silver is the free xbl = you can download content from the online marketplace. Add friends, IM, headset talk.
gold is the one you have to pay for = it has all of these plus the element of online gaming.
I have used up my free gold trail and will be getting the card soon. Basically you click, join xbox live and follow the annoying step by step rules, remember though to say that you are 18 otherwise its a bitch to go through and change parental settings. Thats where i went wrong and had to change everything for the better online experience!