I'm guessing you mean the difference between direct and indirect object pronouns?
It depends on the verb.....
If the noun which the pronoun is replacing would usually be preceded by "à" then you need an indirect object pronoun.
If there is no preposition before the noun, then it is generally a direct object.
Thus;
J'ai envoyé un cadeau à mon frère
This sentence has two objects. un cadeau is the direct object as it is not preceded by a preposition. mon frère is the indirect object as it is preceded by à
So...
Je l'ai envoyé à mon frère
Je lui ai envoyé un cadeau
Je le lui ai envoyé
Make sense?
You have to learn which verbs are always followed by à or de in French, as they don't always follow the English translations. Although this example works in English the same way; I send a present (direct) to my brother (indirect) , there are lots of French verbs which take indirect objects, but in English take direct objects. E.g. Dire quelque chose à quelqu'un, so Je lui ai dit = I told him