Collector numbers first appeared on cards in Exodus at the same time when color-coding of the expansion symbols was added to easily determine rarity. They were pushed through by the then Magic Brand Manager Joel Mick. The first number represents the card's number in the series, the second number represents the total number of cards in the set. The numbers have no effect on game play.
From Del Laugel, Magic Technical Editor:
|“|| When I assign collector numbers to a card set, I start by grouping the cards by color. All the white cards are together at the beginning, followed by blue, black, red, and green, and finally multicolored cards. (The 'WUBRG' color order is pretty standard. It mirrors the pentagon of colors on the back of every Magic card that puts 'friendly' colors together.) After the colored cards come artifacts, nonbasic lands, and basic lands. Then I alphabetize the cards within each of these nine groups by their English card names (a card has the same collector number in all languages). Once the cards are in order, I start at the beginning with 1 and number from there. The second number printed on the card is just the total number of cards in the set.
If you use three-ring binders to store your Magic cards, collector numbers are a good way to stay organized. All your red Mirrodin cards, for example, will be in one place, and it's easy to see which cards you're missing. Many players don't keep basic land cards in binders, though, which is why basic lands are numbered separately from nonbasic lands and put at the very end.
What if a card falls into multiple categories? Well, until the Mirrodin block introduced artifact lands, that wasn't possible. We decided to bundle the artifact lands with the other nonbasic lands for numbering since most players putting together Constructed decks would expect to find these cards in the land section of their binders."
Colorless creatures and spells, like those in Rise of the Eldrazi are numbered first before the colored cards, then the artifacts, nonbasic lands, and basic lands.