Humans are more closely related to chimpanzees than chimpanzees are to gorillas. If chimpanzees and gorillas are apes, then so are humans. That seems to be the general reasoning behind it.
Wikipedia explains it in a lot of detail: http://en.wikipedia.org/wiki/Ape#Historical_and_modern_terminology (which is where I read it)
Wikipedia is a not a primary source. ("Welcome to Wikipedia, the free encyclopedia that anyone can edit.")