>>46812emulator = a translation layer that reads ROMs/ISOs of your games
decompilation = the game itself is reverse engineered so it can run natively on your PC, like a port.
an emulator, as the name implies, is a software program that was designed to mimic the console hardware so that ROMs/ISOs can be initialized on your computer. you can think of the emulator itself as a sort of translation layer that takes the encoded ROM/ISO and allows your PC to make sense of it, and allows you to interact with its contents. a decompilation, on the other hand, is a game that has been reverse engineered to run on natively on your PC, no translation layer needed. it accesses the graphics API and libraries directly like other native games do.
games ran on an emulator are only as good as the emulator's programming, so there may be various problems with the game like graphical glitches and performance related issues. because decomps are reverse engineered, they're pretty much as issue free as any other PC game.