PGA = Process Global Area - it is a heap of memory where Oracle server process stores its private informations (accessiblke only to that particular process). It represents a process state.

UGA = User Global Area - it is a heap of memory where user session stores its information. So it represents a session state. Sort area, for example, is allocated in each session's UGA.

Depending on your configuration (multithreaded or dedicated) UGA can be contained in two various supperheaps of memory.

If you are using dedicated server mode, a process and a session are almost a synonym. Each session has its own dedicated process and one process belongs only to one session. So in this case oracle can allocate UGA as a subheap of PGA.

But if you are using MTS, then each process can serve many sessions. In this case UGA can not be allocated as a part of particular process, it must be allocated from a memory part that all server process can access. That common area that each process can acces is SGA, so in MTS mode UGA for each session is allocated as a part of SGA.