The only time I have seen a .bin file is when working with CD image files. Usually there are a pair of files one is a .bin and the other is a .cue file. Usually the CD creation software looks at the .cue file which tells the program where to find the .bin file. The .bin file contains all the data, and as Allan said it's in binary.
I haven't seen any .bin files in a while. Most images that I see on the net anymore are .iso, .nrg, or, .ccd.