Currently you have a free silver membership which allows you to download demo games from the arcade and market place, Arcade games are mostly in similar to a lot of flash games you get on the web, but they are fun and addictive, you can play a demo of most of them for free and pay a one off fee to completly unlock them.
the Marketplace allows you to download demos of recent game and film releases. This imo is great as it allows you to try before you buy, without having to get a magazine or anything.
The Gold Memebership allows you to further play against other players online in many of the 360 games.
Its worth it for the online gaming alone, the other stuff is just nice to have
its usually about £40 p/a for the gold membership thats about £00.11 a day and trust me as soon as you start playing your favorite games with/against your mates you will wonder why you did not start this sooner.
I have recomeneded this to a lot of people it is so utterly simple to play people.
You have a firends list, this will notify you of who is online and who is offline, you can then tell what games they are playing from this list. If you want to play with them all you need to do is put in the correct game and click on your friend. you can then join them or invite them to your game.
It is a very simple and effective way of building a community. If you play someone in a game and you get along with them then you can invite them to your friends list, if you find they have been obnoxious then you can leave them feedback (it anonamous).
I can not recomend it enough, you recived a demo live subscription with your 360 I seriously recommend you try it out and add some of us to your friends list.
Live is what makes the 360 so good.