Timeline is essentially a scrapbook of your whole life on Facebook. By contrast, Facebook's traditional profile page is more of a snapshot of you today.
Although some people have already voluntarily switched to Timeline, Facebook hadn't made that mandatory. Beginning Tuesday, Facebook is telling some users that they have seven days to clean up their profiles before Timeline gets automatically activated. Facebook is rolling out the requirement to others over the next few weeks.
At some point, even those who haven't logged on to Facebook in a while will be automatically switched.