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.

                            White
                 Green            Blue          
                       red  Black

the colors combine in two ways. friendly colors and enemy colors. 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 Ravnica Block many decks can be built on any color combination.. after the release of Ravnica Block many color combo's were named by the corresponding guild from ravnica:

White+Blue --> Azorius

White+Black --> Orzhov

Blue+Black --> Dimir

Blue+Red --> Izzet

Black+red --> Rakdos

Black+Green --> Golgari

Red+White --> Boros

Red+Green --> Grull

Green+White --> Selesnya

Green+Blue --> Simic

these are no longer known by these names as the idea of all colors combining has become more common.