I advise against Eric's suggestion unless you are certain the systems are of the same model. The operating system build installed on the source computer may not be suitable for the target computer - for example, version 10.6.8 for a Macbook isn't the same thing as version 10.6.8 for an iMac. Builds may also vary between different models of iMacs/Macbooks/etc. Problems may vary from "slight" to "computer doesn't boot anymore and you have to reinstall everything".
For the same reason, attempting to restore a full Time Machine backup onto the target computer may end up with the same results. Migration Assistant, of course, avoids this issue for you.
The basic recommended process is to:
- Make a full Time Machine backup of the source computer onto your USB hard drive: http://support.apple.com/kb/HT1427
- Attach the backup drive to the target computer, then run Migration Assistant on that system.
- Tell it to migrate "from" a "Time Machine backup or other disk". The following page should show you the backup's icon, labeled after the drive of the computer it was taken from. Continuing past that should finally ask what sort of data you want to bring across before getting on with the job.
Once the migration is complete, bear in mind that any user data copied across will NOT be located in the account you're already logged in to. You'll need to use the Apple menu to log out, which'll then allow you to log into one of the migrated accounts.