Phex is a p2p client like LimeWire and it connects to the same network (the Gnutella Network).
It is slightly more complex than LimeWire but has some capabilities that LimeWire doesn't have.
You can get an overview at
Get Phex - PhexWiki
Also please have a look at our guide for new users:
New User - PhexWiki
And besides: Welcome to Gnutella and happy sharing!