All new Macs come with iMovie, which is good for home movies.
For more professional you're looking at using some more expensive (or free, if you know where to look) software. I always use Adobe Premiere Pro, mainly because it is integrates REALLY well with creative suite.
For example Premiere Pro can directly put photoshop PSDs on the timeline and you can do effects on them. It just works really well.
However, I know lots of people use Sony Vegas also, and final cut pro as someone said, but I've never used these. I just know that Premiere Pro is very good.