I Use Bitcoin t-shirt
What the heck is Bitcoin?
"Bitcoin is a peer-to-peer currency. Peer-to-peer means that no central authority issues new money or tracks transactions. These tasks are managed collectively by the network." - Bitcoin.org
So, it's like digital cash without the banks and regulations of other currency like GBP or USD?
Exactly, and you can download the official client from bitcoin.org.
And then I buy this t-shirt to help promote the currency revolution that is Bitcoin?
Right again! If you wish, of course. :)