Even short movies will have file sizes that are probably too large to send by email, So, publishing to the web is usually the best way to go. To send by email you would have to export the movie at a very small size - but it probably wouldn't look that good when opened by the recipient.
If using Share>Media Browser (which I mostly use), you are presented with a range of sizes. For emailing, pick the smallest size. To locate the file, in Finder go to your User (Home) folder Movies/iMovie Projects. Right-click (or control-click) on the name of your project and select Show Package Contents. In the Movies folder you will see your exported movie (or movies if you shared at more than one size).
Copy then paste the movie file to your Desktop or preferred location - don't move or otherwise disturb files in package contents! Now you can use that copied movie as desired, such as copying or moving it to a portable drive for playing through a Media Player.
In iDVD you can simply access the file from iDVD's media browser (click on the Movies tab). Drag the movie to the Theme background.
If you export your movie through "Share>Export using QuickTime" or "Share>Export Movie", you are able to nominate the save location (unlike the Media Browser option).
I'm not sure where the iTunes export saves to (see ADDENDUM). If you have an Apple TV, it's a requirement to Share to iTunes - that's where Apple TV looks for the movie file.
NOTE: Export using QuickTime allows you to customize the export settings, such as format (codec), data rates, size, sound and so forth. But unless you have a special reason to use that method, it's preferable to stick with one of the Apple presets as they are optimized for the set purpose.
ADDENDUM: The export to iTunes is saved in your User (Home) folder "Music/iTunes/iTunes Music/Movies" - unless of course you've moved your iTunes Library to another location.
Message was edited by: John Cogdell - added NOTE
Message was edited by: John Cogdell - added ADDENDUM