Its sometimes hard to type it if logarithm is not natural and base is not 10, especially if base is variable. So anyone know rules how to type?
In their reference, Wolfram|Alpha states the following:
Log[z]gives the natural logarithm of $z$ (logarithm to base $e$).
Log[b,z]gives the logarithm to base $b$.
Michael's answer states this using parenthesis. Note that brackets are formally defined, while parentheses are inferred. Realistically this makes no difference, but for the sake of pedantry.
Additionally, if you search for the term that you need more information on, in this case
log, you can get the definition & documentation by hovering over the shortened definition in the bottom corner:
The Wolfram|Alpha reference, provides amazing insight into these type of questions.