It's not just language learners that use Spanish direct and indirect pronouns incorrectly, but before discussing that, let's look at how we should be using them.

Correct use of direct object pronouns: lo, la, los, las

Lo, la, los, las are the correct direct object pronouns in Spanish in sentences with transitive verbs where we are using these pronouns referring to a masculine singular noun: lo, a masculine plural noun: los, a feminine singular noun: la, and a feminine plural noun: las.

Here are some examples:

Correct use of indirect object pronouns: le, les

Le, les are the correct indirect object pronouns in Spanish in sentences where we are using these pronouns referring to a masculine or feminine singular noun: le or a masculine or feminine plural noun: les.

For example:

Test yourself by studying these lessons: 

• Using lo, la, los, las = him, her, it, them (direct object pronouns)
• Using le and les = [to] it,him, her, them (indirect object pronouns)

However, there are certain parts of Spain and Latin America where this rule isn't followed fully, creating linguistic phenomenons called: leísmo, laísmo and loísmo, which involve the incorrect use (in most cases) of these pronouns.

Let's see each of them in turn.

Leísmo

Sometimes in Spanish you will hear people use the Spanish le/les instead of the correct direct object pronouns lo/la/los/las for him/her/them. This is called leísmo. For example:

Transitive verbs that use a direct object pronoun need one of the direct pronouns lo/la/los/las, not either of the indirect object pronouns le/les.

Leísmo is fairly common in Spain, in some areas more than others. It is often heard and also seen in written Spanish.

This incorrect use of the Spanish le/les is not accepted by the Spanish Royal Academy (R.A.E.) when it refers to her (singular) or them (plural), or when it refers to inanimate objects, but it is accepted in the singular form le when referring to "him".

There is also what is called leísmo de cortesía which is also accepted, when we are using a formal register, for example:

With verbos de afección psíquica, i.e. verbs that affect the mood or provoke some reaction, both types of pronouns (le/les and lo/los) are accepted. Examples of these verbs are molestar (to bother), fastidiar (to bother), entretener (to entertain), amenazar (to threat), aterrorizar (to terrify)...

Laísmo

Sometimes in Spanish you will hear people use the Spanish la/las instead of the correct indirect object pronouns le/les for her. This is called laísmo. For example:

These sentences are using la/las as indirect object pronouns referring to her or them (feminine). As indirect object pronouns these should only be le or les (for masculine and feminine).

Laísmo is not as extended as leísmo. It's common to hear this use of la/las in certain parts of Spain, for example in Madrid and in general in the central and central northern part of Spain. Laísmo hardly occurs in Latin America.

Unlike in leísmo, where certain cases are accepted and normalized, laísmo is not accepted and is considered incorrect.

Loísmo

Sometimes in Spanish you will hear people use the Spanish lo/los instead of the correct indirect object pronouns le/les for him/them. This is called loísmo. For example:

Loísmo is not very extended in Spain or Latin America; you may hear this incorrect use of lo/los in parts of Peru, Bolivia and Argentina, and also in parts of Asturias in northern Spain.

The R.A.E considers this use of lo/los as incorrect. 

It's important to note that these variants already existed in medieval Spain. Also, in Latin America, the influence of some languages like Quechua or Aimara, where there is no distinction of gender, has been an important contributing factor to these phenomena.