What's the difference between "se" and "le"

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This is a little hard to explain.

There are different kind of verbs.

Verbos reflexivos. El verbo afecta al sujeto.
It means that the action affects to the one who does it (Sorry for my English). For these verbs we use "se" before the verb:
Se lava
Se acuesta.

Esto cambia de acuerdo a la conjugación:

Yo ME acuesto (I lay down)
Tú TE acuestas (you lay down)
Él / ella SE acuesta (he / she lays down)
Nosotros NOS acostamos (we lay down)
Ustedes SE acuestan (you (plural) lay down)
Ellos SE acuestan (they lay down).

Verbo transitivo. El verbo afecta a otra persona u objeto.
It means that the action affects someone / something else.
For these verbs we sometimes use "le" before the verb (it's always the same), and sometimes:
Te (you)
Le (him, her)
Les (you plural, them)
before the verb.

Sometimes a "verbo transitivo" works like a "verbo reflexivo":
ME dije a mí mismo
(I told myself)
NOS vi en la televisión
(I watched us on TV)

It's hard to know what kind of verb you use when you're not a native Spanish speaker, you must ask to your teacher.

I know this is a lot of information. Please tell me if it isn't clear.