Difference between transactions and journal
Transactions used to have a very specific meaning. From the New Oxford American Dictionary:


  transactions
  published reports of proceedings at the meetings of a learned society


In this historical usage, it is synonymous to proceedings, i.e. it is a published report of a conference or a set of meetings. Many learned societies have transactions: Philosophical Transactions of the Royal Society, Transactions of the Faraday Society, Transactions of the AMS, etc.. Some other institutions use the term “proceedings”, e.g. Proceedings of the National Academy of Science.

These transactions or proceedings were separate from journals, which do not have to coincide with a conference or meeting. However, with time the customs changed, and many of these reputable publishing venues became independent from actual meetings or conferences. But, because they were reputable (and famous is some cases), the name remained.