UIDs and GIDs in struct task_struct

[Onkar <onkar.n.m@xxxxxxxxx>]

Why are there so many uids and gids in 

struct task_struct {
--
--
    uid_t uid,euid,suid,fsuid;
    gid_t gid,egid,sgid,fsgid;
--
--
}

What is the significance and in which situation we use uid,euid,suid,fsuid and gid,egid,sgid,fsgid ?
any pointers on this ...

Regards, 
Onkar