We use indefinite pronouns to refer to people, things or places without saying exactly who or what they are.
Compare:
| anybody | anything | anywhere | 
| everybody | everything | everywhere | 
| nobody | nothing | nowhere | 
| somebody | something | somewhere | 
Note:  
  anyone = anybody
  everyone = everybody
  no one = nobody
  someone = somebody
No one is written as two words.
When an indefinite pronoun is the subject it is followed by a singular verb.
Indefinite pronouns are singular, but to refer back to them we use plural pronouns, e.g. they, their, themselves, etc.