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Your activity log goes all the way back to the day you started on Facebook. It is a private log of all the activity you have uploaded, contributed to and added to your profile. It is the be-all and end-all of everything that you have done on the social network.
This feature was briefly exposed last year in form of 'Memories', which leaked after someone hit the 'publish' button by mistake. Facebook pulled the social network offline for an hour afterwards to repair the damage.
Clicking the drop-down underneath the top bar allows you to select greater detail. You can see photos, friends, and plenty of other content by category. This allows you to see when you added people as friends, or when they added you. It gives you a full chronology of what happened and when.
Facebook really does mean 'timeline', when it was announced it at the Facebook f8 conference today. You can go all the way back to birth, and add baby photos of when you were a child. You can also add events, life changing or otherwise, such as moving home or going to college.
In this case, you can retrospectively add content to your profile -- even if it was before you joined the social network. Facebook gives you a variety of options to add, or 'other' if it does not fit the category. As you can see here, you can even add very personal stuff such as "having surgery" or "broke a bone" to your profile, to allow others to see as and when things happened during your life.