What DeLay did:

http://graphics8.nytimes.com/images/2005/09/28/politics/29indictgraphic.jpg

Here is the Center for American Progress' explanation - the link is a NYTimes graphic:

In Texas, it is illegal to contribute corporate money to state races. DeLay set up a Texas political action committee called Texans for a Republican Majority (TRMPAC) which accepted $155,000 of corporate cash, exploiting a loophole that allowed it to do so to pay for "administrative expenses." (That's a whole lot of staplers). TRMPAC then wrote a $190,000 check to Terri Nelson of the Republican National Committee with a list of state candidates who should receive the money and dollar amounts each person should receive. The corporate cash infusion into state races allowed Republicans to gain control of the state legislature, placing DeLay's allies in charge of the redistricting process, which eventually strengthened DeLay's stranglehold on power in the House. DeLay doesn't dispute the basic facts, but claims, somewhat inexplicably, that the scheme was legal.