Magic: The Gathering, since it's conception, has always been centered around five colors of magic, or The Pentagram of Colors, each covering different types of magic from necromancy to mind magic to restoring one's health.
White: Protection and Justice Magic, calling mana from the Plains.
Blue: Deceit, Intellect, Water and Ice Magic, calling mana from the Islands.
Black: Decay and Death Magic, calling mana from the Swamps.
Red: Fury, Chaos and Fire Magic, calling mana from the Mountains.
Green: Nature and Life Magic, calling mana from the Forests.
However, these different types of magic are not always confined to specific colors. During the Kamigawa cycle, ice and snow fell upon the mountains of the plane causing it to fit under red magic instead.
When Magic was first designed, all colors had both the colors were disigned in a pentagon shape. this shpe apears on the back of every card.
The colors combine in two ways; friendly color combinations and enemy color combinations. Friendly colors are adjacent on the wheel (Like green and white). Enemy colors are across on the color wheel (like blue and red). These created themes with which the color's themes would compliment or clash with accordingly. Simply looking at the back of any Magic card shows the Pentagram of Colors. Starting with the base color of your choice, by looking to that color's immediate neighbor color determines it's friendly colors. By looking across the pentagram, you find it's enemy colors. As the game has progressed the lines blur. Especially with the release of the Return to Ravnica block many decks can be built on any color combination.. after the release of Return to Ravnica block many color combo's were named by the corresponding guild from ravnica:
White+Blue --> Azorius Senate
White+Black --> Orzhov Syndicate
Blue+Black --> House Dimir
Blue+Red --> Izzet League
Black+red --> Cult of Rakdos
Black+Green --> Golgari Swarm
Red+White --> Boros Legion
Red+Green --> Gruul Clans
Green+White --> Selesnya Conclave
Green+Blue --> Simic Combine
These are no longer known by these names as the idea of all colors combining has become more common.