fMSX is an emulator of MSX, MSX2, and MSX2+ home computers. Originally, it was the most popular for MS-DOS. It is written by Marat Fayzullin. It's not for free.
NLMSX is an MSX/MSX2/MSX2+/TurboR emulator that is based upon fMSX 2.0b made by Marat Fayzullin and is being developed by Frits Hilderink, a former member of MSX Computer Club Enschede in the Netherlands.
If fMSX was the most popular emu for DOS, RuMSX was for Windows. It's still used by many people who looks for comfort of handling, without great pretensions.
Don't you know what it is an emulator? It's simple: an emulator is a program that let you run games and programs of your old computers into the modern PCs, Macs, mobile phones and new generation consoles. You could load the images of those games, whose are available for thousands in Internet.