A Mini Plug is a 3.5mm male connector found on computer headphone, headset, microphone, and speakers, which connects to the computer's sound card. The device you are using determines where the Mini Plug is inserted into the sound card. For example, with a computer headset plugs into the line out and microphone jack and speakers plug into the sound out jack. The picture below is an example of a mini plug connector on the end of a pair of headphones.
Some higher quality headphones use a 1/4" (6.3mm) stereo plug. To connect these headphones to the computer, you must purchase a 1/4" (6.3mm) female to 3.5mm male adapter.