Since BIP144, transaction information consists of two components (regular information and witness information). There are additionally two distinct transaction serialization codecs:

• The legacy serialization format, which shops model quantity, transaction inputs, transaction outputs, and locktime.
• The prolonged serialization format, which additionally has a flag/marker to point the prolonged format is in use, and has witness information for each transaction enter.

The prolonged format should be used if witness information is current. The legacy format should be used if no witness information is current.

The txid is the double-SHA256 hash of the serialized transaction in legacy format, that means that any witness information needs to be stripped out (because the legacy format doesn’t help witness information). If the transaction had no witness information within the first place (as pre-segwit transaction do), the txid is simply the hash of the serialized transaction.

The hash (or wtxid) of a transaction is the double-SHA256 hash of the serialization together with potential witness information. If the transaction accommodates no witness information, this similar to the txid.