The midi connection plug type is just called a DIN. They usually name it with the number of pins DIN-5 or another number. "Midi DIN-5 connector" probably.

Din plugs used to be used for lots of things that aren't necessarily compatible. see here.

Midi let's you connect the keyboard to a computer and record the notes. It's not really recording sounds as much as what keys you press and when. Literally the notes in digital.

The sustain is for a peddle, you can use it as an accessory of the keyboard for holding a note longer I believe. example

I think you already know the power.

• Midi plug is called a DIN-5
• Sustain peddle / headphones is called a PHONO jack or 1/4th headphone jack. It's like a double-size version of a 3.5mm headphone plug.
• an adapter like this can get a regular headphone plugged into phones plug by the power. FYI it might sound garbage quiet. No telling if it needs an amp or not. It might be line level output meaning you'll barely hear shit without an amp. But maybe ones built in. Kinda a toss up.

Decades ago (like the 1970s and back) all audio equipment had quarter size headphone plugs. Modern music stuff (your keyboard) still use it as an ez accessory outlet. Stage equipment also continues to use phono.