Time machine... whats a full backup?

I am using TM but I prefer to tell it to do backups rather than the default way.
I always do a TM backup before updating/installing any software.
Did a recent upgrade to 10.5.5 (not good - too many issues) but needed to get back to my previous setup.
Went through the restore complete system process but when it asked which TM backup I wanted to restore, it didn't offer me the latest, which was about 30 minutes previous. It said it only offers full backups.
Can anyone explain what the difference is between the backup I did manually and a full backup.
Thanks

N

just do what most of us do, let time machine do its own thing. I tried to do it my way, then some months ago decided to reformat the external HD and let it do its own, my system went down, I inserted the install discs again, and did restore from backup and everything went brilliantly. Trust the Apple system