A log is simply a tool to track the time you spend, both on a units or hours per week basis, and remembering how you spent your time. The Pomodoro Technique is an excellent way to track how you are spending your time. The Keep it Collared excel or Google spreadsheet is a great way to track your hours worked per week.
I use the Keep it Collared spreadsheet, since I develop it. I also use the Pomodoro timer, although I don't feel the need to track what each pomodoro is for.