You just clock in when you start and clock out when you're done. No paper logs or notebooks, no trying to remember what / when you worked. You can even keep track of specific jobs if you need to. You can also log time manually in case you forget to clock in.
It's especially handy since I work from home and start and stop often throughout the day.
I can clock in and out from my iPod Touch or from my computers. All of them are synched together in a WiFi area. Check it out if you have need of something like this. It's free for a single person but has a price if a company uses it for its employees. A+ recommendation from me.