Switches Vs Routers and EGP vs IPG

[Betsy Brennan]

Can you help me understand when a router would be used instead of a
switch?   

Also, I need to understand the differences between EGP and IGP.
I know that IGP is for exchanging network info within a network
and EGP is for use between Autonomous systems, but what other
differences are there?
Respectfully, Betsy B.