The property of transitivity or intransitivity applies specifically to verbs. Every verb belongs to one of these two categories, though on occasions some verbs can switch between the two types.
He barked. (intransitive verb)
Julie eats an apple. (transitive verb)
Note: Some verbs can be either transitive or intransitive, depending on context. They're sometimes called ambitransitive or verbes labiles.
For example, the verb parler (to speak) can be used as an intransitive verb, on its own, with no object:
Je parle. - I'm speaking.
But it can also be used as a transitive verb, followed by an indirect object:
Je parle à ma mère. - I'm speaking to my mother.
Attention: For some verbs, the auxiliary (être or avoir) in compound tenses and sometimes the meaning can change depending on its transitivity. Look at the verb monter:
Je monte au deuxième étage. - I'm going up to the second floor.
Je monte les escaliers. - I'm going up the stairs.
Je suis monté au deuxième étage. - I went up to the second floor.
J'ai monté les escaliers. - I went up the stairs.