I use litecoin-qt, but my guess it'll be the same for Bitcoin or any other crypto coin.
I created a new wallet, added an address and received the coins I expected. Then I created another wallet on different PC added another (non default) address, and sent some of the coins from first address mentioned to the second. I used 'sendfrom' command.
Now both wallets show exactly what I would expect, however on any online block explorer I see something strange. When I enter the first address, I see the transaction with the initial amount, but then another row that withdraw that amount. Clicking on the transaction I see that the second address is credited with the amount I sent, and the rest goes to an unfamiliar address.
What does it mean or what am I missing? Where's this 3rd address coming from? I thought I know how things are working with cryptocoins, but now I'm completely lost.