Direct objects are called complementos directos and come after transitive verbs, which require an object to fully convey their meaning.

Él escribió esta carta. – He wrote this letter.
Ayer vimos una película muy buena. – Yesterday we watched a great movie.

Note: The direct object (complement directo) comes with the preposition a when we talk about a person. Check the difference.

Vimos un perro. (We saw a dog.)
Conozco Madrid. (I’ve been to Madrid.)

Ayer vimos a María. (Yesterday we saw María.)
Conozco a tu madre. (I know your mother.)

Indirect objects are called complementos indirectos and refer to the beneficiary of the action indicated by the verb. Indirect obejcts are introduced by the preposition a or para.

Él escribió a Juan. – He wrote to Juan.
Compramos un regalo  para Luci. – We bought a present for Luci.

In Spanish, pronouns also play the role of direct and indirect objects. In this case they’re called pronombres complemento. Take a look at these sentences:

Complemento directo

Él escribió la carta. (He wrote the letter.)
Él la escribió. (He wrote it.)

Ayer hicimos el emparedado. (Yesterday we made the sandwich.)
Ayer lo hicimos. (Yesterday we made it.)

Complemento indirecto 

Él escribió a Juan. (He wrote to Juan.)
Él le escribió. (He wrote to him.)

Compramos un regalo para Luci. (We bought a present for Luci.)
Le compramos un regalo. (We bought her a present.)

So, in the direct object sentences we use la (for feminine) and lo (for masculine). In indirect object sentences we use le for both masculine and feminine.

Note: When the direct object is masculine (person), we can use lo(s) or le(s).

Yo vi a Juan. (I saw Juan.)
Yo lo / le vi. (I saw him.)

Yo vi a los niños. (I saw the kids.)
Yo los / les vi. (I saw them.)

Esperen la parte 02 muy pronto.