What is the Steam gaming platform? What is Steam?  
Why should I buy the game on Steam instead from normal retailer? 
Can a game bought via Steam played independently?
What is Steam?

According to them, they are  The Ultimate Online Game Platform. What this actually means is that they are a portal for you to buy, play and update games. With certain benefits. But “steam” is just that. They sell games (more on this later), they offer you a community where you can have Friends, Chat with them, unlock achievements, etc. If you’re a user of an iPhone/Pod/Pad that has GameCenter in it, think of Steam as a pseudo GameCenter.

Why Steam and not normal retailer?

The advantages of Steam are a few, and I couldn’t list them all because some things may not be better for everybody, but let’s just say that the strength of the platform and what makes it different from buying a boxed game could be summarized in a few bullets (in no particular order):


Very easy to buy a game (and discover new ones)
Instant buy, start downloading, no delivery costs.
Automatic updates without having to worry
If you buy a game on your Macintosh and one day decide to play it on your Windows computer, provided that the game is supported in that Platform, you can continue your game there, as if nothing has happened. You re-download the game in your Windows machine and voilá.
Friends, Achivements, Progress… Steam as a platform sports game achievements that you unlock by playing.
Discounts, sometimes you will get “cheaper” versions of the games or promos that you can take advantage of. 
More, I’m sure there are other advantages but think of an AppStore for games with a plus. 


Can games be played independently?

Yes and no. I don’t believe you can copy that game over another machine (tho you could probably install steam on that machine and copy the game… I haven’t tried this). But you can play offline if you don’t have Internet. 

Bonus: Is it worth?

If you have a Mac and like gaming, it’s definitely a big plus, as a lot of games get ported and put into the Steam network. If you’re in Windows, I believe the ease of use and “point and click” nature of the service make it worth. And also the fact that you don’t have to pay to use it. Did I mention it’s a free service? ;)

But Does it have Cons?

Some invaluable readers pointed out a few cons, which I will update (and I encourage everybody to contribute):


When you start it, it automatically updates, giving you the sensation that the client is hang. Also if your network speed is not optimal, this process can demand a lot of time.
If your network connection is not good, downloading three gigabytes of game might not be the best idea, given that you usually can’t start playing until almost most (if not all) the content is downloaded. 
In order to play offline, you have to log in, and tell steam “I’m going offline”, which kind of voids the possibility of an unexpected loss of connectivity or forgetting to do it before disconnecting for example. 
I didn’t mention this, but it’s important to note that not all the Windows games are available on Steam and not all work on OS X. Although decent amount (and growing) catalog is available, not every title is there. 


UPDATE: There’s a dedicating gaming community in Stack Exchange for that. Don’t hesitate asking gaming questions there too, those guys may seem like gamers, but they aren’t playing at all ;)