Transaction confirmation
Transaction processing is a function of mining.  Each time a block is found observed transactions become included in the block chain and get one confirmation.  The Bitcoin protocol is designed to have an adjusting difficulty to ensure that blocks are not generated at a speed greater than that>   This ensures the gradual distribution of new coins into the economy as well.  So long story short this is the model that Bitcoin uses and it is one of the few shortcomings of performing transactions.