NCO BHO & Intrusion Prevention? What do these do?

I just purchased Norton Internet Security 2010.  It seems to be running fine.  It asked me if I want to enable Norton Web Safe and I said yes.  If I am correct, this enables NCO BHO and Intrusion Prevention.  Please tell me the functions of each of these features.  Are they really necessary?  Do they use a large amount of system resources?  Do most people run them or not?  Will they slow down my web browsing?

 

Thanks in advance!

Gary