Your rating indicates your skill level. Ratings can help you recognise an evenly matched opponent. You can learn more about the individual ratings formulas.
Ladders is a mechanism for ranking players.
A player's rung is his or her position on the ladder. When a game in a ladders room is completed, the ranking of players is modified to move the winner up and the loser down. Your position in the ladder will not change if you play a game in a regular room. You can learn more about how ladders function. Your ladder rung and rating are two independent metrics.