Xbox Live has changed a bit for the Xbox 360. There are 2 membership levels (Gold and Silver) and they each allow you to do access different features and at different price levels. Here's everything you wanted to know about the difference between the 2 levels of membership and which one is right for you.
Xbox Live Silver (free)
Create your gamer profile: Ok, you get a unique ID to identify yourself on the Xbox Live network. Sweet.
Create and maintain a friends list: You can chat with Xbox Live friends just like any Instant messaging program.
Access To Xbox Live Marketplace: You'll be able to download game demos, skins to customize the look of Xbox live, game content, and arcade mini-games. Some downloads will be free (demos, trailers) and others will be premium (arcade games).
Xbox Live Gold (as low as $50/year)
All the Silver level benefits PLUS:
Online gameplay: You can only play games on Xbox Live if you have a Gold membership.
Exclusive Xbox Live Marketplace Content: A few extra downloadable perks for being a gold member.
Those are the basic differences between Silver and Gold.
Bottom line: to play your Xbox 360 games online, you'll have to purchase a Gold membership.
The details about the different memberships can be found here:
http://www.xbox.com/en-US/live/memberships/
UPDATE: Our friends at Teamxbox.com have put together
a little guide about the different media downloads for each launch game (thanks stealthTT for the tip!)