I see someone already mentioned that the transaction log is a write-ahead log, which, IMO, is its most important distinguishing feature. The transaction log is a core operational strategy employed by SQL Server. It is not a record of "what happened", not an added feature, and not in the slightest optional.

I think that most people who complain about ballooing log files are confused by the term "log." Let me try some analogies here.

A paper check register is a type of log. You write a check, then record it in the register (don't you hate the people who hold up the line doing this?). But, you might forget to do that. You might also log the wrong amount. And the check doesn't suffer for it--it's still paid as you wrote it. Your check register is only valid if you are accurate and diligent. Not only that, but your bank couldn't care a bit if your register is wrong--it has no bearing.

That's a write-after log. Take action, record it. And it's what most people think of when you say "log."

Now, suppose that you don't write checks by hand, you only print them from a finance program. First, you enter into the program all the details about the check. Then, the program uses those details to print the check. The record of the check in that program is a write-ahead log. The check can't exist without that log entry, and the log entry was used to create the actual check.

SQL Server uses a write-ahead transaction log. A committed transaction goes to the log FIRST. The lazy writer uses the LOG as its source of modifications to make to the actual database files at the next checkpoint. Before that checkpoint occurs, your committed data exists only in the log. Therefore, the log is always accurate and complete because the data wouldn't be there without it.

And, I have to take issue with the MS Access origin of the transaction log. I was working with SQL Server 6.0 at the same time as MS Access 2.0 was hot. SQL Server has used a write-ahead log for its core operation for longer than MS Access has existed! I think any feature copying was the other way around.