Hi Uri,

It all depends on what abstraction level we want to be. :-)
At a higher abstraction level, I think it suffices to say "empty the log" (or "truncate", whichever
in English gets the message through). I prefer to say "empty", since I believe it makes it clearer
that the file isn't made smaller.

There are of course much more details what happens when you BACKUP LOG. I stay away from those
details when I see a basic question and I suspect the OP has limited experience with transaction log
internals - like virtual log files. The reasoning is that more details will only cloud the message.

As for the details, this is how I believe it works. Anyone is of course free to add and correct,
below if off the top of my head:
A VLF can be in four states:
A. Never ever used
B. Used, and all log records are prior than the last checkpoint and have also been written to disk.
C. Used, but some log records are more recent than last checkpoint and/or not written to disk.
D. The active log (where the current log record, "head of the log", is).

BACKUP will not actually delete anything from the log file, it will just allow for re-use of the
virtual log file. Basically turning C above into B. A and B can be re-used (overwritten whenever the
head of the log moves to this virtual log file). These have status 0 in DBCC LOGINFO, while C and D
have status 2.

Now, above takes some thinking and some visualization in the head (or whiteboard). Which is why I
tend to stick with the higher abstraction level for basic questions. :-).