Find below a rather technical description of some verb structures in Spanish. This is not intended for beginners (...or highly advanced learners), although anyone is welcome to have a read through.
1) Verbs that can take a direct object are called transitive. When this direct object is a specific person (or something personalised), it takes the preposition 'a'. Objects can be replaced by the direct object pronouns 'lo, la, los, las (many native speakers use 'le' and 'les' instead sometimes, but this is not always standard):
Veo un camión. Lo veo. (I see a truck. I see it) Veo una casa. La veo. (I see a house. I see it) Veo a tu hermano. Lo veo (I see your brother. I see him) Veo a tu hermana. La veo (I see your sister. I see her)
2) If the object matches the subject, we talk about reflexive (direct object) pronouns:
Me veo (a mí mismo): I see myself Te ves (a ti mismo): You see yourself Se ve (a sí mismo): He/she sees himself.
3) When someone does something to others, and the rest do the same, and of course, back to this person, we talk about reciprocal (direct object) pronouns, like in 'each other'. I give, and others give to me:
Nos vemos. We see each other (literal translation) = see you (correct translation) Se ven. They see each other.
4) If the indirect object matches the subject, we talk about reflexive (indirect object) pronouns:
Le lavo la espalda a mi hijo. 'I wash the back to my son. (non-reflexive) Me lavo la espalda. 'I wash myself on my back. (reflexive)
Of course, in English you'd say something like 'I wash my son's back.' and 'I wash my back.'
5) Sometimes a pronoun is used to indicate completeness. Sometimes it is hard to translate these:
Me comí una manzana. I ate (up) an apple. Me bebí la cerveza. I drank (up) the beer. Me fumé el cigarro. I smoked the cigarette. Me leí el libro. I read (the whole) book.
In the examples above, both the apple and the beer were totally consumed. It is very unusual to omit these pronouns (and it sounds a bit strange), but you cannot use them if there is no specific amount to consume:
Me comí arroz (wrong sentence. you cannot use 'me')
6) Intransitive constructions don't have a direct object. Intransitive verbs like 'gustar' use the indirect object to specify who 'likes' what (or whom):
Me gusta el chocolate. 'To me, is pleasing chocolate.' (I like chocolate)
English has a few verbs like this. One of them, 'disgust' is directly related to 'gustar', and it also uses an indirect object to indicate who doesn't like something (we don't say 'who disgusts something', like in Spanish):
Me repugna el chocolate. Chocolate disgusts me.
People say 'I like chocolate', but not 'I disgust chocolate'. The second one is the construction used for 'gustar' in Spanish.
7) When the indirect object matches the subject, we talk about reflexive (indirect object) pronouns:
Me gusto. I like myself (compare: I disgust myself)
8) When you do something to others, and others give to you, we talk about reciprocal (indirect object) pronouns:
Nos gustamos. We like each other Se gustan. They like each other