Rotisserie scoring is based on standings in each category. For example, in a 12 team league, the team with the most cumulative home runs gets 12 points, 2nd place 11 points, 3rd place 10, etc., down to 1 point for last place. The same is done for all the scoring categories. You add up your team's points for a total, which is compared against the other teams' for the overall standings. In categories like batting average your team's total average (total hits/total ab's) is used--which is why a high average hitter that plays a lot and gets more ab's is worth more than a high average hitter with few ab's. That is also why Adam Dunn isn't rated very highly
Rotisserie standings change as the season goes along and the winner is determined at the end of the season since everything is cumulative.