Emulators or ROM sets are the best things that can run any game right online without any issues at all.
If you are a beginner then it might be confusing in the beginning which is why you should download emulator by comparing them with the others. It will make sure that you are downloading the right one for your use.

Factors to be considered
At the time of downloading there are plenty of factors you need to consider like at first you need to identify your requirements. If you want to play old school games then you should go for the ROM sets or on the other hand if you want to play games of PlayStation or X-Box then you must go for the emulators.
Keep one thing in mind that emulators are different for different console games so you must first read out the reviews given by the people as that will be going to help you out in downloading the right one for your use.
High end combination you can use
Emulator and ROM library games can work together which will be going to provide you old school gaming with advance technology. The process is simple as you simple need to download the emulator and the ROM games files. Now those files can run with the help of emulator easily without any problems.
Also there are some games those who require high-end specs so you need to make sure of that thing before downloading the game and also good specification will be going to help in running games without any lag or delaying problems on PC.