As far as I can tell the Steam software is an Accessory Tray program that gives you the ability to download an encrypted 'preload' copy of HL2 (as well as find game servers for all their games, buy other games, chat, get software updates and news). Once it goes public you will be able to pay for the game online through Steam, and will get the tool to extract HL2. Then you will be able to play it just as if you bought a copy in the store.
I've got my copy of HL2 preload sitting on my HD. Now I'm Jonesing for some info on when it's going to be released...
From Steam FAQ...
"Steam is is the fastest and most secure way to purchase a Game from Valve Software. When you purchase a game through Steam, the game is registered directly to your Steam Account.
You will not need to enter a CDKey to retrieve your game, you will only need your to login to your Steam Account to get access to your games. Games purchased through Steam do not expire and are a one-time charge (not a recurring fee).
Games purchased online do not include a CDKey, a physical CD or a manual.
When you sign up for Steam, you create an account that contains all of your Steam games. You can use your Steam account on any computer, which allows you to play your games at home, at work, or on a friend’s computer.
If you buy a new computer or need to re-format your old computer, you can keep using your old account—Steam will automatically download all of your information to the new computer.
Although you can use your account on any computer, it can only be used on one computer at a time.
CDKeys are special serial numbers printed on retail copies of Half-Life and other games in the Half-Life family. Before Steam, this key was used as an anti-piracy measure, and became the user’s unique ID code when playing the game online. With Steam, the CDKey can be used as a “proof of purchase” code that will grant your Steam account access to some of the games available through Steam.
Each CDKey may only be used with one Steam account. Once used, it is permanently bound to that account, and may not be used with another. Deleting the account will not free the CDKey to be used again.
A Steam Account may have more than one CDKey registered to it. For instance, if you have an account with a Counter-Strike CDKey registered to it, and you purchase Half-Life 2 in the store, you may use the HL2 CDKey on your Steam account to grant access to HL2.
If you purchase a game online through Steam you do not get a CDKey, instead you get a ProductKey. ProductKeys are registered immediately to your Steam Account so you can begin downloading the game right away. You do not get a CDKey number when you purchase a game online."