P.L. 104-130 took effect on January 1, 1997, giving the President the ability to veto "specific spending or certain tax provisions of legislation", rather than having to veto an entire bill. this was good because it allowed the President to simply veto the parts of a bill he disagreed with instead of having to veto an entire bill, simply because he disagreed with a single part of it. A line-item veto could be used to cancel "in whole (1) any dollar amount of discretionary budget authority, (2) any item of new direct spending, and (3) certain limited tax benefits. In exercising this authority, the President had to determine that such cancellation would (1) reduce the federal budget deficit, (2) not impair any essential government functions, and (3) not harm the national interest."
on June 26, 1998, the Supreme Court, in a 6-3 decision, ruled that P.L. 104-130 (the line-item veto law) was unconstitutional. had it not been struck down, it would have expired at the end of 2004 unless Congress "voted to extend it's provisions."
direct quotations taken from http://rs9.loc.gov/home/line_item_veto.html