What's the difference/relation between pid,tgid,pgid,sid?

[Cristian Tapus <cristian.tapus@xxxxxxxxx>]

Can somebody explain to me or point me to some explanation of what the
pgid and sid are and how they are related to the pid and the tgid?

I am trying to understand how the pid hashing works and how I can
change the pid of a process while it's running.  I know I need to
do some __detach_pid and then attach it again under a different pid,
but I'm not sure what to do about the rest of the pid types (PGID,
SID)...

Cheers,

Cristian.