Splitting a transaction and sending some of the coins back to the sender. Necessary for some currencies due to the design of the protocol.
With Bitcoin and its relatives, the protocol specifies that when a transaction spends an output of a previous transaction, it must spend the entire amount. If the sender doesn't actually want to spend that much, the new transaction includes an extra output, sending the remaining coins back to a (new or existing) address belonging to the sender. This is called change by analogy with the real-world process of paying a large note for a small purchase, and receiving the difference back in smaller denominations.
More information can be found at https://en.bitcoin.it/wiki/Change.