Generally speaking, a "boot sector virus" is a virus that inhabits the boot sector of a floppy disk or a hard drive. It doesn't necessarily have anything to do with the data files on that disk. (Although it could be conceivably copied to any location on the disk, it isn't likely to copy itself from there.)

The boot sector is the part of the disk that gets loaded automatically into memory when you boot your computer. If you have an infected disk in your floppy drive when you boot, the virus propagates itself to any disk you format from that point on, until you disinfect your system. At least, that's the essence of how boot sector viruses work; they could be programmed to do any number of other nasty things as well.