What is the difference between Trading Volume and Transaction Volume? Looking at the Bitcoin Statistics summery from Blockchain.info

there are Trading Volume and Transaction Volume listed.
I understand that transaction volume represents the amount of bitcoin used for transactions, eg. user sends bitcoins over to another user.

What about trade though? How is the value for trading value determined?
Trading, in this context, refers to the exchange of Bitcoins for fiat currency (dollars, euros, etc).

Major exchanges report on the number and total value of trades made on their platform.  Presumably the numbers on blockchain.info come from aggregating that data.