I transferred money few times between my checking accounts in different banks (Capital One, Ally, Chase, BoA etc). I did not see any fees taken yet and I am not sure if any will be. I checked fees for checking accounts in this banks and it seems all banks have some fees - $15 or $30. But usually they say "domestic wire and international wire". I am not sure if I used "wire" and what it means. Is transferring money between your own account means using "wire"? If not, what is the difference between the way I transfer it between my accounts and wiring? Why I did not get any fees for transfers?
EDIT: So I guess I transfer by default using ACH transfer according to the first answer. Why ACH transfers are free and wires are subject to fees?