A spell is either a card that has been cast and thus placed on the stack, or a copy of another spell. A card is only a spell when it is on the stack; in most other zones it is simply a card, or a permanent when on the battlefield. All card types other than lands are types of spells; even permanent cards are normally cast as spells before becoming permanents. Spells exist as game objects, and their rules determine what interactions and effects are possible between the casting of the spell and the spell taking effect by "resolving".
Frequently, non-permanents are collectively referred to as "spells" in informal discussions. This includes sorceries and instants, along with the obsoleted mana sources and interrupts. This doesn't line up exactly with the game rules, which state that even permanent cards (except lands) are spells from the time they are cast until they resolve. This usage is instead based on the fact that typically, non-permanents only have gameplay effects while they're on the stack as spells, while permanents are relevant only after they enter the battlefield (and are no longer spells). Thus, whenever instants and sorceries are collectively referred to as a set theme or mechanic, it falls under the umbrella of "spells matter".
The original card type names "Summon" (instead of Creature) and "Enchant [type]" (instead of Enchantment - Aura) put more emphasis on the idea that the cards represented spells which resulted in permanents, rather than being permanents inherently. The modern type names create a clearer identity as permanents, which makes the casual terms "spells vs permanents" more distinct.
In some other contexts such as deckbuilding, spells can be referred to in the context of what they contrast to. "Spells vs. lands," asks about the land count ratio; "creature vs. noncreature spells," informs to what lengths the deck wishes to play to the combat step; "subtyped/tribal spells vs. those spells not subtyped", asks functionally what proportion of the deck satisfies a certain tribal playstyle.
Mages use many different practices to channel mana, including chants, screams, and ritual motion. The concept of magic, unlike physics, has no unbreakable laws; spellcasters can use their spell do virtually anything. 
He imagined these cards separated into five colors, each with their own strategies and flavor of play: Red attacks aggressively, blue plays tricks with spells, green plays slowly but utilizes powerful creatures late in the game, black weakens enemies while strengthening allies, and white produces small creatures and protects life totals. Decks can be built using any combination of these colors. And each color fits into a larger lore mirroring the fantasy worlds in the role-playing games Garfield and Adkison grew up with.
Many new players are confused by permission spells, probably because the cards are so unique in what they do. At various early points in my career, I remember trying to use Counterspell to destroy a permanent already in play (not legal), and trying to use Counterspell to stop my opponent from playing a land (not legal either).
While there are times when memorizing the technicalities of Magic is overly tedious, in the case of permission spells, it's very helpful. The following terms will definitely be familiar, but their exact definitions may or may not be.
Stack: The stack can feel like a scary concept when you first learn about it, but it's really not so bad. It's simply where spells and abilities wait to resolve. When you cast a spell or activate an ability, it goes on the stack, and both players get a chance to respond to it. If they choose not to, the spell or ability resolves as normal.
Spell: A spell only exists while it's being cast, while it's on the stack, and while it's resolving. You can use permission to remove a spell from the stack, before it has had its effect on the game. Instants and sorceries are spells as you're casting them. Creatures, planeswalkers, enchantments, and artifacts are also spells as you're casting them, but not once they're on the battlefield.
Land: Lands are permanents, but they are never spells. Playing a land is a special action and does not use the stack. In other words, it happens immediately, and cannot be countered or responded to.
A problem arises, however, if you do somehow fall behind. Permission spells cannot remove a creature or planeswalker from the battlefield, so if one slips through the cracks, you'll need another way to answer it.
However, they can fall short against blisteringly fast decks like Mono-Red or White Weenie. You won't be able to counter a turn-one creature (at least not in Standard), so you'll be starting the game with a big problem. Either you'll suffer a ton of damage from that creature, or you'll have to tap out to answer it with a removal spell or creature of your own. At this point, your shields will be down, and your opponent can resolve another threat. You'll have to tap out to answer that one, and if you're not careful, you can find yourself in a vicious cycle where your permission spells are largely useless.
Traditionally, blue-based aggressive decks are cold-blooded killers of slow, controlling decks. They get ahead early, and then sit back on counterspells to prevent the control decks from clawing back into the game.
This nightmare scenario can be avoided with the presence of a modest amount of permission. If blue is known for two things, it's its permission spells and card drawing, and the two make a deadly combination. Once you've taken control of the game, your card-drawing spells allow you to see a large portion of your library, and if there are even a few permission spells in there, you can use them to make sure the game doesn't slip through your fingers.
First off, permission spells in your sideboard won't be a liability against those super-aggressive decks that are going to put you on the back foot right away. Too many permission spells in your main deck would be.
Second, just like with removal spells, after sideboarding you can tailor your permission suite perfectly to beat the person sitting across from you. Disdainful Stroke is great against ramp decks like Green Devotion. Negate is excellent against controlling decks like Esper Dragons. Annul is stellar against Blue-Red Artifacts and Constellation decks.
Finally, many matchups slow down a bit after sideboarding, and you may need extra answers to whatever your opponent is bringing in against you. For example, you might normally consider a card like Ultimate Price to be your best weapon against Green Devotion, but if your opponent is siding in Nissa, Worldwaker against you, you might need to sprinkle in a few permission spells in order to be better prepared.
In other words, balance is key. Make permission spells one extra tool at your disposal, but don't lean too heavily on them. It's great to have a small number of counters in either your main deck or your sideboard, but you'd want a special reason before you'd play with more than two or three.
We've touched on playing with permission spells in our previous discussions of tempo. Early in the game, you'll sometimes need to pull out all the stops in order to avoid falling behind on tempo. Don't be too proud to Dissolve a Foundry Street Denizen, if that's what the situation calls for!
Imagine you're playing a Blue-Black Control deck with a modest amount of permission in your deck (we'll say five counterspells), but you also have a lot of removal spells and a few creatures. When you play this deck against Abzan, it's important to use the right tools for the right jobs. If your opponent casts Anafenza, the Foremost, you can consider letting it resolve. After all, this is a card that you can answer just fine with a Hero's Downfall, Murderous Cut, or Languish. A morph that could be a Den Protector, however, will be troublesome if it hits the battlefield, so maybe you ought to save your Dissolve for something like that.
Permission spells do have one weakness that can sometimes be exploited: They must be cast in a narrow window. If your opponent passes the turn with mana open for Dissolve, they've made a commitment of mana, and they must either use their Dissolve, or let it go to waste. If you suspect that your opponent is holding up a Dissolve, you can choose to cast your second-best spell, or perhaps not cast anything at all if you have that luxury.
Permission spells are an important, complex, and challenging part of Magic. However, there's no reason to be afraid of them. Understand their strengths and weaknesses. Learn how best to use them yourself. Learn when you can make them inconvenient for your opponent. These are skills that will serve you well throughout your Magic career.
Each turn proceeds in the same sequence. Whenever you enter a new step or phase, any triggered abilities that happen during that step or phase trigger and are put on the stack. The active player (the player whose turn it is) gets to start casting spells and activating abilities, then each other player in turn order will too. When all players decline to do anything more and nothing is on the stack waiting to resolve, the game will move to the next step.
For this reason, I want to increase the consistency requirements for spells with higher converted mana costs. Specifically, I will require 90% consistency for 1-mana cards, 91% consistency for 2-mana cards, 92% consistency for 3-mana cards, and so on, up to 95% for 6-mana cards.
Hence, in the updated analysis, for a card costing N mana of a certain color with converted mana cost M, I consider the probability of drawing at least N colored sources by turn M on the play conditional on drawing at least M lands by that turn (after mulligans). So I will assume that you cast spells on-curve. This means that for Wrath of God, I now consider the probability of drawing at least two white sources by turn 4 on the play conditional on drawing at least four lands (after mulligans). 2b1af7f3a8