you can back it up.
You buy the game, that is bound to your account, then you can install it, uninstall it, mod the files, whatever, it is just a base for accessing the games and that, it is all stored on your system.
If you want to back up the game files you can, just right click the game in your games list and click backup game files... That will let you back it up to your hard drive, another hard drive, and external drive, a USB device, DVD, CD whatever and it will backup the game files and saves.
I'd imagine steam and all of its stuff will always be there. If Valve goes out of business (the people that develop and run steam) then you will still be able to get to your account and steam and that