What is the jet engine at tail of the plane?

Many current jets have their engines on the wings instead of at the tail. Some older passenger planes (L-1011 Tristar, DC-10, MD-11) had engines in both locations. The planes that have only tail jets are the MD-80 and the older Boeing 727 and DC-9.

The small exhaust ports on some passenger aircraft are from auxiliary power units (APU's), which are small turbine generators that provide power independently of the main engines. You can see these on small Lear jets as well as on huge 747's.

(photo at related link)