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.

There is no difference.

In particular, IEEE Transactions on Software Engineering is a journal, as are all the other IEEE Transactions.

From IEEE:

IEEE Transactions generally contain major manuscripts approximately 8 to 10 printed pages or 24 to 30 double-spaced pages.

IEEE Journals follow the same length criteria as Transactions, but often are focused on selected topics and more specialized areas of interest.

IEEE Letters are generally short papers of approximately three to four printed pages or nine double-spaced pages. "