(article by Gerald Erichsen from spanish.about.com)
Se is undoubtedly the most versatile of the Spanish pronouns. As you learn Spanish, you will come across se used in a variety of ways. For the beginner, it isn’t necessary to learn all those ways, although it can be helpful to be introduced to its various uses to help avoid confusion when you see it used in a way you haven’t studied yet.
Here are the major uses of se, along with examples:
As a reflexive pronoun: This is its most common use. As is explained in the lesson on reflexive pronouns, such pronouns indicate that the subject of a verb is also its object. In English, this is usually accomplished by using verbs such as “himself” or “themselves.” Se is used as the reflexive pronoun for third-person uses (including when usted or ustedes is the subject). Some verbs (as in the final two examples below) can be used reflexively in Spanish even though they aren’t translated that way in English.
* Examples: Pablo se ve por el espejo. (Pablo sees himself using the mirror.) Los padres no pueden oírse. (The parents can’t hear themselves.) Rebecca se perjudica por fumar. (Rebecca is hurting herself by smoking.) Benjamín Franklin se levantaba temprano. (Benjamin Franklin got up early.) Se comió los tacos. (He ate up the tacos.)
As the equivalent of the English passive voice: By using se, particularly when discussing inanimate objects, it is possible to indicate some sort of action without indicating who performed the action. Grammatically, such sentences are structured in the same way that sentences using reflexive verbs are. Thus in a literal sense, a sentence such as se venden coches means “cars sell themselves.” In actuality, however, such a sentence would be the English equivalent of “cars are sold” or, more loosely translated, “cars for sale.”
* Examples: Se abren las puertas. (The doors are opened.) Se vendió la computadora. (The computer was sold.) Se perdieron los llaves. (The keys were lost.) Se prohibe fumar. (Smoking is prohibited.)
As a substitute for le or les: When the indirect-object pronoun le or les is immediately followed by another pronoun that begins with an l, the le or les is changed to se, apparently as a way to avoid having two pronouns in a row beginning with the l sound.
* Examples: Déselo a ella. (Give it to her.) Se lo dijo a él. (He told it to him.) No se lo voy a dar a ellos. (I’m not going to give it to them.)
The impersonal se: In some sentences, se is used in an impersonal sense with singular verbs to indicate that people in general, or no person in particular, performs the action. When se is used in this way, the sentence follows the same pattern as those in which the main verb is used reflexively, except that there is no subject to the sentence that is explicitly stated. As the examples below show, there are a variety of ways such sentences can be translated to English.
* Examples: Se maneja rápidamente en Lima. (People drive fast in Lima.) Se puede encontrar cocos en el mercado. (You can find coconuts in the market.) Muchas veces se tiene que estudiar para aprender. (Often you have to study to learn.) No se debe comer con prisa. (One ought not to eat quickly.)
A final note: Se shouldn’t be confused with sé (note the accent mark), which is usually the singular first-person present indicative form of saber (“to know”). Thus sé usually means “I know.” Sé can also be the singular familiar imperative form of ser; in that case it means “you be.”