Because & because of

1) the difference
Because is a conjunction. It is used at the beginning of a clause, before a subject and verb. Because of is a two-word preposition, used before a noun or a pronoun. Compare:
We were late because it rained, (*not ... because of it rained.)
We were late because of the rain.
I'm happy because I met you. I'm happy because of you.

2) position of because-clauses

Because and its clause can go after or before the main clause:
I finished early because I worked fast.
Because I worked fast, I finished early.

Because-clauses can stand alone as answers or after hesitations, but not usually in other cases:
'Why are you crying?' 'Because John and I have had a row.'
I don't think I'll go to the party after all, actually... Because I'm feeling a bit tired.
Note that after reason we usually use that or why, not because