(example) Vedo Mario. Lo vedo. (I see Mario. I see him.)

Mangio la pizza. La mangio.

_

In contrast, indirect object pronouns answer the question "To whom?" or "For whom?". The indirect object (as a noun) is always preceeded by "a" or "per".

(example) Do la pizza a Mario. Gli do la pizza. (I give the pizza to Mario. To him, I give the pizza.)

Compro una cartolina per Maria. Le compro una cartolina. (I'm buying a postcard for her.)

A Luisa piace l'appartamento. Le piace l'appartamento. - (The apartment is pleasing to Luisa. The apartment is pleasing to her.)

The indirect object pronouns mi ti ci vi are the same as the direct object pronouns. In Italian, indirect object pronouns differ from direct object pronouns only in the third-person:

to / for him - gli

to / for her - le

to / for you (formal) - Le

to / for them - gli

(You may hear someone use "loro": to / for them - loro) (it comes after the verb)