Difference between User Exit & a Functional Module

Dear all,

Please help me in understanding ,what's an User Exit & a Functional Module in

standard SAP.How do we define both of them.

1.When can/cannot I use an user exit?How to check for which T codes in PP

Module an User Exit can be used or present.

2.Can I use only a Functional Modules for Z developments or also an user Exit for

Z developments?

Please explain it in a simple way.

It will be a great help for me,Expecting for your help.

Regards

Mangal