How does a SIM work for t-mobile?

I planning to get this: http://www.t-mobile.co.uk/...ym/extras/

i want to get a new iphone with no contract off amazon. Can i insert it into the iphone and all i will have to pay monthly is $21 for that plan?
please don't call me stupid at all or anything insulting. Sorry i'm not smart.

Added (1). Could this work on an iPhone?

Well remember, the SIM card is where the service is coming from. Even in the US, my T-Mobile doesn't even charge that price. This makes me wonder "what's the catch?" I'm looking at it and it looks like you get charged by the minutes. It says that all calls to number 02, 03… Etc are inclusive. These inclusive numbers are for the costs below in the PLAN COST. I have no idea how that works but I assume that once you insert your sim, they will track down how many calls/ minutes you use.