"PC" is a much more generic name than it used to be. But it used to refer specifically the IBM PC. If a computer was advertised as "PC compatible" it meant that it would run the same stuff as an IBM PC - they were PC clones. Over time, as the PC compatible market became swamped with clones, the term "PC" started to be less specific to IBM and more of a generic term for actual IBM or clones. This trend continued once Windows became popularized. Now when you need to differentiate one type of computer from another, it's done by what OS it runs rather than the maker of the hardware (although in Apple's case, since there are no Apple clones, the maker and OS are used interchangeably most of the time).