This is what a player's Steam game library looks like:
It's all pretty simple: on the left are the games you have bought or installed, and in the center are detailed information about the game and the launch button.
Gaming community
Almost every game has its own community (group) where players post all sorts of screenshots and videos of the game. Any player can capture an interesting moment in the game and share it with the community, where other players will comment on it.
Staying in touch with friends
Steam has one-on-one chat, group chat and voice chat (like Skype). You can also invite any friend to a game, or join one yourself by choosing "Join Game" in the menu, and you don't need to ask for or type in any server IPs; everything is done automatically.

Your games are available on any computer
All your games can be accessed from another computer; all you need is to log in with your Steam account. You buy a game only once. If you uninstall a game, wipe your computer or do anything else, you won't lose the game itself. Games are tied to your account and can be downloaded again. Also, thanks to Steam Cloud, game files (saves, configs and so on) can be synced with the Steam server, in which case all your saves will be available on other computers.
Many Valve games have now been ported to Linux, so some games can be played on Windows as well as on Ubuntu, Mac OS and other operating systems. And you don't have to buy the game separately for each platform.
Protection against game file modification
The Steam version is protected against changes to the game files. If for some reason game files have been changed or deleted, the Steam client will automatically re-download or replace them. This is very handy if some server has changed your menu or added something unwanted: in that case you can delete the unneeded files without worrying that the game won't start, because if you delete something important, Steam will fix it.
Achievements
Some games (Half-Life 2, Black Mesa and others) have achievements based on the tasks you have completed; pirated copies don't have that.
Game prices
Licensed games aren't all that expensive, and sometimes (during sales) they cost next to nothing. Steam sales aren't exactly a rare thing. Any fan of Half-Life or Counter-Strike should own licensed copies of the games.
Some games are sold as collections (anthologies), and if you buy games together rather than separately, you can save quite a bit.
Don't forget that by buying games you help Valve keep developing them. Maybe Half-Life 3 still hasn't come out because of all the people who downloaded pirated copies instead of buying the games!
Other advantages
Licensed games update automatically, so you don't have to hunt for patches to bring your games up to the right version. And if you don't want your games to update automatically, you can turn that off.
A licensed copy lets you play on Steam servers, where the number of cheaters is practically zero.
The community right in the game. While you're in a game, you can access the community or visit a website without leaving it:
Some mods don't run on pirated copies and require the Steam client to be installed.
You can download Steam for free from the official website http://store.steampowered.com/ via "Install Steam" in the top left corner.
1. Не хочу неожиданных обновлений от разработчиков, которые "лучше знают", как я должен играть.
2. Клиентская часть Steam на моей старенькой машинке кушает слишком много ресурсов. А оптимизировать клиент у разработчиков руки не доходят.
Все вышесказанное является личным мнением, если хотите ИМХО и написано не для того что-бы кого-то отговорить, убедить, что-то доказать и т.д.
Half-Life FAQ - ответы на многие вопросы
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