|"It is a HP Pavilion 700"|
700 is the generic series number, the actual model number is probably on a label on the backside of the case.
The power supply (aka PSU) is a box within the computer case. It converts the AC input into several different DC outputs. The main 3 voltage rails are +3.3v, +5v, & +12v. If any one of the 3 fail, the PSU has to be replaced.