What is Minecraft Multiplayer?
Multiplayer is a way to play Minecraft with other people. It's a way of connecting to a server and playing with other people. You can join a server that is already running, or you can create your own server and invite others to join you.
What is the difference between Servers and Realms?
Servers are run by other people, and you can join them to play with other people. Realms are run by Mojang, and you can invite other people to join your Realm to play together. Realms are limited to 10 players, while servers can have many more players.