Here's how it works: every day, when you arrive to work, start up gtimelog and type "arrived". Then start doing some activity (e.g. reading mail, or working on a task). Whenever you stop doing an activity (either when you have finished it, or when you switch to working on something else), type the name of the activity into the gtimelog prompt. Try to use the same text if you make several entries for an activity (history helps here -- just use the up and down arrow keys). They key principle here is to name the activity after you've stopped working on it, and not when you've started. Of course you can type the activity name upfront, and just delay pressing the Enter key until you're done.

There are two broad categories of activities: ones that count as work (coding, planning, writing proposals or reports, answering work-related email), and ones that don't (browsing the web for fun, reading personal email, chatting with a friend on the phone for two hours, going out for a lunch break). To indicate which activities are not work related add two asterisks to the activity name.

GTimeLog displays all the things you've done today, and calculates the total time you spent working, and the total time you spent "slacking". It also advises you how much time you still have to work today to get 8 hours of work done. There are two basic views: one shows all the activities in chronological order, with starting and ending times; while another groups all entries with the same into one activity and just shows the total duration.

At the end of the day you can send off a daily report by choosing Report -> Daily Report. A mail program (Mutt in a terminal, unless you have changed it in ~/.gtimelog/gtimelogrc) will be started with all the activities listed in it. My Mutt configuration lets me edit the report before sending it.