When you hit a key on an electronic piano, it plays a synthesized sound. (By synthesized I mean it's not an actual instrument doing the sound, it's a computer.)
Depends how high and hard you crank up the electric piano. Also depends on what kind of pianos they are. But acoustically a normal piano will have the same volume if not more than an electric one.
Grand pianos and acoustic uprights are different. Grand piano: A key is a long piece of wood which is balanced on a rail in the middle of the keybed. Depressing the key makes the opposite end lift...
When you play a piano you hit the keys. When you hit the keys you trigger little hammers that hit strings in the piano which vibrate and the pitch depends on the diameter and length of the string.
An electric piano requires you to plug it in so you can use it. You can play the piano even if there is no electricity. Some keyboards also comes with recording devices and other tunes and sound...