What is a Y pipe?

A y-pipe is an exhaust component that brings together the exhaust gases from both sides of a "V" type engine into a single pipe. The Y designation implies 2 exhaust inlets going into a single outlet.

There are also reverse Y pipes. These are used primarily with a single turbo setup with only 1 exhaust outlet from the turbo. The reverse Y splits the single exhaust stream into two separate pipes for a dual exhaust configuration.

The 350Z and G35 come standard with a y-pipe, but it is very heavy and restrictive. (SEE OEM Y PIPE)

What is it made of?

Y pipes come in various materials: steel, stainless steel, and titanium are the favorite flavors. Plain steel is the cheapest, but over the long term is susceptible to rusting. It is also the cheapest. Stainless steel is the most popular. You get a pretty shine, no rust problems, and it lasts a very long time. Pricing on stainless is very good compared to titanium. For the top end consumer, there is the option of titanium y pipes. These are very expensive (~$1000), but are extremely light. For the weight conscious Z owner, this opportunity cannot be passed up.

What else are they called?

Sometimes they are referred to as mid-pipes or down-pipes. Also you will sometimes see reverse y-pipes mentioned when talking about single turbo kits working with a dual exhaust system.