Personal "a"

When the direct object of a verb (except tener) is a person, it is preceded by a.  It isn't used if a number precedes the object though.  The pronouns alguien (somebody), alguno (someone), nadie (nobody), and ninguno (no one) require a as well, when used as the direct object.

Veo a Juan.  I see John.
Conozco a tu amiga. I know your friend.
Veo a alguien.  I see somebody.