Right a selector is a patch that enables you to select between Pal/Ntsc
A trainer is a patch which allows you to give yourselves extra lives, shots E.t.c.
The most common patches out there are PPf patches and you need PPf-o-matic to apply them you can download that from www.megagames.com then all you do is apply the PPf file to the Bin file that you create when you extract the psx game to your hard drive and burn back to cd. Simple as!!!!

Hope this helps
Deano