When you send a transaction to the Bitcoin network you also pay a small fee. This is typically 0.0001 BTC per transaction but it varies. The computer in the Bitcoin network that includes your transaction in a block receives all of this fee. (MultiBit does not receive any of this fee.)
The rules for the calculated fee are a little complicated but mainly depend of how much space the transaction takes up in the block. You can see the size of the transaction by viewing the 'Transactions' tab. Select the transaction, right click and choose 'View transaction details. . .' Typically you pay 0.0001 BTC per 1000 bytes of transaction size.
TIP: you can reduce your fees paid by avoiding the receipt of lots of very small amounts of bitcoin. These tend to create large transactions when you spend your bitcoin and incur larger fees.