Nebula is described as 'arcade emulator. Specifically it emulates the CPS 1 & 2, Neo-Geo, PGM (PolyGameMaster) and Konami systems. Features include multiplayer, netplay and cheat code support' and is an app in the Gaming Software category. There are nine alternatives to Nebula for a variety of platforms, including Windows, Linux, Mac, Xbox and Android. The best alternative is RetroArch, which is both free and Open Source. Other great apps like Nebula are Mame (Free, Open Source), Nostlan (Free, Open Source), FB Alpha (Free, Open Source) and RetriX (Free, Open Source).
What is RetroArch? RetroArch is a modular multi-system emulator system that is designed to be fast, lightweight, and portable. It has features lacking in some emulators frontends, such as real-time rewinding and game-aware shading. What systems does it play?
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.
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.
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.
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.
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!