Paid Time
So far I've seen two basic ways that the time you are paid for is accumulated. You might be paid for all the time you spend with an active Internet connection or for all time spent doing various Internet related activities. The last case might only count the time you spend in a browser surfing the Web or it might also include email or chat time. In any case you have to have the bar program running.

All sites have some method of verifying activity. Methods used vary but include smart bars that accumulate time when a web browser is the active window and there is keystroke or mouse activity within a certain time limit, bars that time out occasionally and require you to click on them to resume accumulating time and bars that time out if the mouse hasn't moved over them recently. All bars have some kind of indication of whether you are on or off the clock. In general the sites that allow you to accumulate time whenever you have an active connection to the Internet and have their bar running will produce the most revenue. It depends on the rates a site pays, how much time you can maintain an active connection and on your Internet usage habits.