On some weekends you will get to play free on XBL silver, it's like when you get HBO for a weekend to get you hooked, if you want to play games online you will have to buy XBL gold subscription, Here is what you get with the two.
Xbox Live Silver level
Create an online Gamer Profile
Access the Xbox Live Marketplace
Engage in voice and text messaging
Talk to a single friend at a time using voice chat
Receive video messages from Gold level members
Access massively multiplayer online games (additional fees may apply)
Xbox Live Gold level
Paid level of service
All the features of Silver level, and additionally:
o Play multiplayer games online
o Video chat
o Multiplayer online tournaments
o Participate in Xbox Live online programming, such as Game with Fame, Play and Win, and Prime Time activities