Emulators allow one to run applications and games in an environment for which they where not originally designed
Play Nintendo 64 games on a home PC, Play Nintendo Entertainment System games on a Sony PSP, or Play old arcade games on a Linux or Windows PC.
Frontends are GUIs (Graphic user interfaces). These are generally designed for command line programs. They may also be more detailed GUIs (pronounced Gooeys) that allow more features to be easily accessed for a particular program or multiple programs.