To a native speaker, both sentences have the same meaning and are interchangeable. However, the meaning is only identical if you only want to mention which company employs you. As Jakub mentioned, if you want to stress the specific location where you are working, then you must use the second sentence. You could say, for example: I work at apple. (means "I work for the apple corporation", but does not indicate where you work, so it has identical meaning with "I work for apple") I work at the apple store in Portland. (indicates where you work, but also based on the meaning also communicates that you work for apple corporation) I work for apple at their corporate headquarters in Cuppertino. (Here you are combining both sentence 1 and 2 to describe which company you work for, and the specific location)

In my opinion the first sentence "I work for apple" can be strictly used to describe who is your employer. The second sentence is related generally to the place you are working at certain time, in other words the place where you are working is at "Apple Corporation"