OK Ask Mr Google

http://www.google.co.in/search?hl=en&source=hp&q=autotransformer&meta=&aq=f&oq=

Results 1 - 10 of about 198,000 for autotransformer [definition]. (0.26 seconds)

http://www.google.co.in/search?hl=en&source=hp&q=power+transformer&meta=&aq=f&oq=

Results 1 - 10 of about 2,540,000 for power transformer. (0.32 seconds)

was it that difficult?

http://en.wikipedia.org/wiki/Autotransformer

Typically, a power transformer will consist of (at least) two electrically isolated coils wound on a common iron core.

An autotransformer consists of a SINGLE coil on an iron core. Typically input voltage is applied across the full coil. the autotransformer output voltage is developed between a third terminal and one of the input voltage terminals. It is the magnetic equivalent of a three terminal restance potentiometer.