Making an orkut application or using orkut you should know these basic terms used by orkut.

Terms:
uid- user identification
tid - thread identification
aid - the album identification
pid - photo in the album

Uses:
lists, testimonials, settings and updates use xid whereas all other
features use uid or gid.

gid- for images

Random and unity:
gid - not random
uid - random

gid an uid both are unique per user
GID is the number of your avatar picture

These will useful when you want to target users via social application or vb application