-body or –one?

There is no difference between somebody or someone, anybody or anyone, nobody or no one and everybody or everyone. The forms with –body are very informal.

Some- and any-

The differences between somebody and anybody, something and anything, somewhere and anywhere are the same as the differences between some and any.

The forms with some- are more common in affirmative clauses. The forms with any- are more common in negative and interrogative clauses.

Did anybody come? (NOT Did somebody come?)
There is something under the bed. (NOT There is anything under the bed.)
I don’t want to buy anything too expensive.

Singular verbs

Everybody, anybody, somebody etc are usually used with singular verbs.

Everybody needs to be loved. (NOT Everybody need to be loved.)
Somebody has let the cat in. (NOT Somebody have let the cat in.)

Somebody and some people

Somebody refers to only one person. When the reference is to more than one person, we use some people.

There is somebody at the door. (One person)
There are some people at the door. (More than one person)

Pronouns referring back to somebody, anybody etc

The pronouns they, them and their are often used with a singular meaning to refer back to somebody, anybody etc.

‘There is somebody at the door.’ ‘Tell them I am busy.’ (More natural than ‘Tell him / her I am busy.’)

Anyone and any one; everyone and every one

Anyone means anybody. Any one means ‘any single person or thing’. There is a similar difference between everyone and every one.