Magic: The Gathering - The Basics, Part 1:

In Magic: The Gathering, players take on the roles of opposing Planeswalkers: god-like beings who can, under their own power, travel between worlds and - as their name implies - other planes of reality. As a Planeswalker, you tap into the land itself to summon little dudes - called "creatures" (and yes, human is just one of many creature sub-types) - to fight your opponent. Creatures also serve the dual role of blocking your opponent's creatures when they attack you, in return.

Both players start the game with a sixty-card deck; a seven-card opening hand; and twenty life points (which, as you might well expect, can go either up or down during the match). When one player's life points are reduced to zero (or any of the other victory conditions - albeit rare [such as running out of cards to draw] - are met), they lose the game!

The most basic resources in the game are land cards. Roughly one-third (and sometimes more!) of any good Magic: The Gathering deck is made up of lands. In this regard, deck design is somewhat easier than it is in other collectible card games...

Players "tap" (turn sideways, in order to show that it's been used this turn) land for one of five different colors of "mana" (a term you're undoubtedly familiar with if you've ever played the Diablo video games) - which is magical energy that's required to cast spells (comprised of all the other non-land card types). There are five different basic land types (although of course, there's plenty of other non-basic lands, too) - each with its own associated color of mana. Naturally, certain colors work better together than others - and while I've listed enemy colors, they are not in fact impossible to play in tandem (just more difficult).

The colors are, in order:

White mana
Basic land type: Plains
Allied colors: Blue and green
Enemy colors: Black and red
Largest creature type: Angels. Every set has at least one angel in it (and sometimes more) - most of them rare.
Most ubiquitous creature type: Human.
Strengths: Healing, damage prevention and redirection; lots of small creatures that are fast to bring into play (which have been nicknamed "weenies" by players)
Weaknesses: White has trouble, both with being able to deal much damage, and to deal it fast enough.

Blue mana
Basic land type: Islands
Allied colors: White and black
Enemy colors: Red and green
Largest creature type: Pretty much non-applicable, since it's a built-in counter-balance to blue's overall power in other aspects of the game. While blue does have a few large creatures, most of them have severe built-in drawbacks to their use.
Most ubiquitous creature type: Wizard (be they Merfolk, Faerie, etc.)
Streghths: Countering opponents' spells entirely; drawing lots of extra cards. Blue is often considered to be Magic's most powerful color.
Weaknesses: It is also the slowest. It takes longer to get the ball rolling with blue than it does with any other color.

Black mana
Basic land type: Swamps
Allied colors: Blue and red
Enemy colors: White and green
Largest creature type: Demons, Vampires and Djinns. Those three are among the rarest creature types in the game, though - sometimes years go by without new instances of the same!
Most ubiquitous creature type: Zombies, Skeletons etc.
Strengths: Forcing opponents to discard from their hands; bringing their own stuff back from the discard pile
Weaknesses: Black is the only color to have no way to deal with two powerful card types - Artifacts and Enchantments.

Red mana
Basic land type: Mountains
Allied colors: Black and green
Enemy colors: White and blue
Largest creature type: Dragons
Most ubiquitous creature type: Goblins
Strengths: Dealing damage directly to an opponent (bypassing blocking creatures entirely); destroying cards that are already in play
Weaknesses: Red often sacrifices caution for speed - many of its spells damage their players as much (sometimes more) as they do their opponents!

Green mana
Basic land type: Forests
Allied colors: White and red
Enemy colors: Blue and black
Largest creature type: Too numerous to list! Green is the color that's known for its big creatures.
Most ubiquitous creature type: Elves
Strengths: Fast mana production; lots of extra mana production; big creatures
Weaknesses: Green is the most average of Magic's five colors - but what it might lack in flashiness, when compared to the others, it more than makes up for by having no real glaring weaknesses...

There are also multi-colored/"gold" (the color of their card borders) spells. These require at least two, and possibly as many as five different colors of mana to cast! They typically combine the strengths of their respective colors, albeit usually not quite so powerful as in their uncombined forms. For example, a multi-colored black and blue spell might force your opponent to discard cards, while you yourself draw an equal amount of cards. Gold's largest creatures are the Elder Dragons.