Limit Number of Process Per User

[Trevor]

What file contains the information to limit the number of processes that a user
account can have running at any given time?  

What would the actual entry look like in this file, to limit the number of processes to 11,
for a user account named "trev"?

What error message would I see on the terminal when this limit has been exceeded?

[Chetan_Tiwary_]

Ok I waited for someone to pour their knowledge, but now it is my turn to showoff :

The configuration file is :  /etc/security/limits.conf

trev           hard    nproc   11

If the limit has been exceeded, then you may see this error in /var/log/messages :

error: fork: Resource temporarily unavailable

or something like :

fork of child failed: Resource temporarily unavailable