I took 737 mini (two day) ground school about 10 years ago. I believe the commercial jets' main electrical buses are 3-phase 110/200 volt 400Hz AC, created by the starter-generators on the turbines. It's relatively straightforward to tap across two of the terminals and get 110VAC, but it'll be at 400Hz, meaning not everything designed for 110VAC/60Hz will be able to handle it.

(Also, some older private aircraft, such as mine, use a 12VDC standard.)