Since can act as an adverb or a conjunction. In either case it can mean from a certain time in the past until the present, or after a certain time in the past. When used as a conjunction it can be used as a synonym for because.
Because is a conjunction which can either introduce an explanation or reason for something. It is also a synonym for since.
Traditionally there was a distinction between the two; however, they are interchangeable now. The one caveat is since’s dual definitions. Sometimes in a sentence, there may be ambiguity as to whether you are using the time meaning or the reasoning meaning. Because only has the reasoning meaning and may be more clear in those instances.
Side note: Because carries its own ambiguity when it follows a negative. ‘He didn’t walk the dog because it was raining’ could mean the rain kept him from walking the dog, or that the rain was not the reason he didn’t walk the dog. To have your meaning be clear, it is best to use a difference sentence construction or a synonym for because.
Amazon’s stock is up 400 percent since the dot-com crash and more than 19,000 percent since the company went public in 1997. [CNBC]
It has since become one of the signature holiday decorations in the town, inspiring Stacy Howard, an office assistant with the town, to write a song, “The Lights of Blue,” in honor of Plouff and other fallen police officers. [Winston Salem Journal]
The prices at Alhayat Global Synergy women’s fashion house in the Sky Memorial Complex have gone up because the Nigerian currency, the naira, has lost 20% of its value in the last three months, as a consequence of the oil crisis. [BBC]