'Because' is typically used to describe the reason behind something. e.g. "You go to school because you like to learn." 'For' is used in a variety of ways, including to address something to someone: e.g. "This letter is for Susan." It can also be used in the same manner that 'because' is used, though this is a more formal, literary way of speaking: e.g. "We can't go outside, for it is raining." 'Since' can be used to describe the way something has been from a certain point in time. e.g. "Since the car accident, I haven't been able to walk." "It's been raining since Friday." It can also be used similarly to 'because,' such as in the following: e.g."Since you're going out, can you grab me a coffee?" "Your car must be nice, since it is new." 'As' is typically used to compare two things. e.g. "It's as hot as a desert out there." 'As' can be used as a substitute for 'because,' but this is not used very often in spoken English. e.g. "As you're sixteen now, you can get your driver's license." 'As' is a very common word in English, I recommend looking at this page for all the ways 'as' can be used. :) http://dictionary.reference.com/browse/as?s=t

They are all conjunctions, used to connect two clauses. All four of them *can* be used to explain something: but the use of "for" for this is the least common. I needed to clean my room, because/since/as/for I had friends coming over. All of them work. "For" is the least common here. The other three are used in this kind of context all the time. "Because" is perhaps used the most common. "Because" also works much better in a sentence that starts with it: Q: Why do I have to clean my room? A: Because I said so. We would not use the other three words in place of "because" here.

Because = we use this when giving a reason/ or answering a question. Typically in a response with a "why" in it.