Leaderboards are used to compare yourself against other players who play in similar tournaments that month and provide a reward for each of the top finishers for each leaderboard. For any tournament, if you play at least 50% of your hands you are awarded points toward any qualifying leaderboards. You can find all promotional and monthly leaderboards on the Promotions page.
You can easily tell which promotions have a leaderboard associated with them, as they will have a Leaderboard chart below the description of the promotion which shows the dates for the leaderboard, the last update, total number of players, player rank, position change, player name, leaderboard score, and chip prize for each rank once the promotion is complete.
The majority of leaderboards only take into consideration your top highest point earnings from qualifying tournaments each month. The number of tournaments that count toward each leaderboard is shown on the specific page for that leaderboard, and is referred to as a 'cap'. Once you have exceeded the cap, only the highest points earnings (your top finishes equal to the cap amount) are totaled for your position on the leaderboard. The points are always counted for your Statistics page (even once you're over the cap amount), but the highest points earnings limit was created out of fairness for those that cannot play in a large amount of tournaments each month.
To see how many tournaments you have played in that count toward the cap, as well as your highest, lowest, and average point totals for the leaderboard, you can hover your cursor over the colored pie chart to the right of your name on the leaderboard. This will show the information listed below so you can see exactly what your stats are for that leaderboard.
The monthly leaderboards run from 00:00 ET on the first day of the month until 23:59 ET the last day of the month, however, some special promotional leaderboards may last for a shorter time (and will be explained on their specific promotion page).