For new people in the world of emulation let us make one thing clear, frontends are not emulators. What frontends are is software that will allow you to access all your roms/games from an convenient interface and allow you to start the appropriate emulator that will run those games.
The more basic frontends just privide a game browser of the roms you have and allow you to launch the emulator. More advanced ones grab data, usually, from online databases and provide you with things like screenshots and info for that specific game you are looking at. That results into a more complete experience.
Depending on what you want to to you may end up using a frontend as a basic emulator launcher and that is perfectly fine if that is all you need. Other more advanced users build full arcade style cabinets that house a small computer which boots to the frontend directly, the end result is a more unified experience that is more similar to running digitally downloaded games in a game console or Steam "big picture" mode.
Selecting the frontend you want to use might be a daunting task if you are a new user which is why we suggest doing some research, seeing what features they offer and if they cover your needs. Below you will find a list of the more popular ones. We highly recommend that you check each one's site so you may see which one is the one you need.