RetroArch is a graphical frontend for emulators, game engines, and media players.
MAME stands for Multiple Arcade Machine Emulator. When used in conjunction with images of the original arcade game's ROM and disk data, MAME attempts to reproduce that game as faithfully as possible on a more modern general-purpose computer.
FB Alpha or FBA as it is commonly known is an emulator of arcade games, that is, it takes the program code, graphics data, etc., from an original arcade game and emulates the hardware to make the game run in it's original form.
Kawaks is a streamlined Neo-Geo/Capcom (CPS1/2) emulator that works well even in low-end computers and is compatible with Kaillera Netplay. You can assign macros to specific keys to perform otherwise complicated supermoves!
A front-end or user interface (UI) is the presentation and interaction layer of an app or website. Most modern emulator apps have basic game list UIs that you can interact with using a mouse/keyboard but not a game controller.
RetriX is an emulator front end for UWP, on all the hardware platforms it supports: it serves the same purpose as RetroArch but is built from the ground up to integrate as well as possible with Windows.