General purpose video editing software

Like Ian C. mentioned, Quicktime is the only thing pre-installed. You can do the basics: rotate, scale, and merge clips.

iMovie is the consumer standard. It comes with the iLife suite. It has an easy to use interface and can create some pretty fancy and professional looking videos. The current version has a tool to make trailers for home movies, includes great control over the audio, has facial recognition, and supports the AVCHD format.

Final Cut Pro X is the prosumer application for video editing. It is the successor to Final Cut Express 4 and Final Cut Pro 7. It, like iMovie, is a paid application. It has a important feature which is not in iMovie: multicam editing. Final Cut Pro X allows you to overlay multiple video tracks, then edit them simultaneously. It also is a 64-bit application.

Pre-installed? QuickTime is really the only thing pre-installed. You can rotate and merge video clips with it, but not much else.

You can buy, for very little money, the iLife pack of applications. You'll find that it's pretty common for Mac owners to buy this one. This bundle comes with iMovie and that's the entry-level video editing application most Mac users know.

Just to note that iMovie can be considered to be a pre-installed App when you buy a Mac, it is merely not included with OS upgrade disks.