What is the transaction ID? I have sent a Bitcoin amount to someone who wants the transaction ID. What is this? Is it the hash shown on the LHS of the transaction?
Transaction ID usually refers to the 2xSHA256 hash of the transaction serialized as it would have been transferred in the network. This is also the ID as peers refer to them when broadcasting through the network.

Notice that due to transaction malleability a transaction may have multiple IDs.