Good Old Games is DRM-free. It's one of the big selling points that they are known for. Some of the games have been specially modified to run on modern operating system. Some just have an emulation layer applied to them. (usually DOSbox) All of them come with a custom installer to automate the setup process.
You purchase a game on Good Old Games. It is now in your GOG account. You can download the files for the game as many times as you like, and install the games on as many computers as you want. Each game comes with all of the original files that came with the original release. (so you could run them with other emulators like SCUMMVM if you really felt like it) You are free to backup your GOG games on CDs, DVDs, external HDD, thumb drives, whatever you please. There is no additional client software, like Steam, you just download each individual game from the GOG site.
GOG isn't strictly necessary. You could pirate any of the titles available on the site easily enough. But it is an excellent way to financially reward someone for archiving and distributing these classic titles. And with the kind of prices they're throwing around in this sale there really is no excuse. I've only been using GOG for a brief time, but have enjoyed their service immensely. I'm going to be keeping a close eye on this summer sale.