It is a gathering of people with purpose of socialising them. There are numerous social events. Bachelor's party, house party, get together, club events, etc. are some of them.
The ultimate goal for the people is to get together and share joy and fun. Some social events are intended to spread a message among local communities. Examples are literature festval, political gathering, technology festivals etc.
Social events can be invitational or open door. They also can be either family based or locality based. Wedding, Birthday parties are family based social events while school gathering, festival celebration are locality based social events.