Like what do they do and do they come stock on a car or only after market?

 

a Y pipe is used to connect 2 manifolds/headers to a single exhuast pipe,basically what comes on single exhuast v8/v6/flat 4 cars.

Also there are aftermarket Y pipes that eliminate CATS.

X pipes are used in aftermarket dual exhuast system,and are used to syncronize exhuast pulses,and tend to kill the sound some. Some duals also use a H pipe for somewhat of the same affect as the X.