What role was played by the Electoral College in the 2000 Presidential election?
A very significant one indeed; for the College determined who became the next President.
For one of the few times in U.S. history, the president who won (George W. Bush) did not receive the most votes.
This is due to the structuring of the Electoral College system in America. I won't go into detail, but basically each state has a certain number of votes called electoral votes. The number of these votes is determined by the population of the state (that is, the more people, the more votes). The purpose of these votes is in choosing the next president; a candidate needs at least 270 votes to win. In the extremely, extremely rare case of a tie, Congress decides on the next president.
For example, in my home state of Connecticut, we have 7 electoral votes. The candidate who receives the most votes in my state gets those seven votes, so he or she only needs 263 more to win. Unfortunately, because we have so few votes, most candidates don't really care about our state ;(. However, this changes when it comes to a big state like California or Texas, where losing could cost you the whole election. In 2000, Democratic candidate Al Gore won more votes than Bush, but Bush won the important states with a lot of electoral votes. As a result, he won the election despite having less votes from the actual American people.
As an interesting side note, there is a lot of controversy surrounding the 2000 election. Some people claim that Bush used his influence in Florida (his brother was the governor) to rig the election. Had Gore won Florida instead of Bush, Gore would have become the president. There were recounts, and petitions, and a whole lot of speculation, but in the end Bush won by about 500 votes. Pretty darn close.