Can a bitcoin transaction be reversed? If I sent bitcoins to someone, but then want to cancel this transaction after it was already made (like a chargeback on a credit card), is this possible with Bitcoin?
No, a bitcoin transaction which has already been included in a block on the longest chain cannot be reversed.  This is by design as Bitcoin is intended to behave much like cash, with recipients being assured that the money is now permanently in their possession after a transaction has been included in a block.