Steam is a digital rights management system.
In order to play Civ5, you have to create a Steam account.
Steam offers also some other services, it's basically also a giant online shop for games, offer friends lists, allows match making, allows you to redownload the game, even if you lost the disc, and some other stuff.
But if you have a game from Steam, you'll have to let Steam authentificate this game online, to see that it's not pirated. If you try to install Civ5 on a computer without Internet, then you're unlucky, that's not possible. You are also not able to resell a game after it was once activated.