Steam is a platform that manages, updates and allows you to purchase games in digital format (download).
It's account based so everygame you purchase is tied to your account which means you won't be able to sell it as a 2nd hand as it wouldn't work.
The games you get from steam are installled either by downloading it but if you purchased the game from the shelves in a game store you can install it from disc as well, in both cases they'll be added to your library in steam which then automatically keeps your game up to date with the latest release of patches and updates.
Modding shouldn't be a problem with Steam at least from personal experience, also more and more titles gain the feature called Mod Workshop which makes it easier to do so (search, install and if you like to mod yourself share your work)
Downloading or getting the Disc is up to you if the game has Steam as it's DRM it doesn't matter. If you have a slow internet getting the disc would be adviced.
Pros:
-Games are kept up to date
-Friendlist to make it easier to play multiplayer with friends
-From time to time steam has grand sales that'll save you some cash on some great titles you wanted to get.
-No disc in drive required, once it's installed you can shelf the disc and launch the game without it.
Cons:
-Once purchased the game you won't be able to resell it as the game is tied to your account.
-Maintenance done to the steam servers may or may not kick you out of the multiplayer session your in (depends if the multiplayer of the game uses Steam as it's primary means of hosting game sessions)
but those imo are a small price to pay for a great service overall, if you can't find the game in local stores or online, Steam is the easy solution to obtain those titles.
HunterICX
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