Here you can find the long explanation about the Elo Scoring System used here on
Lichess.orghttp://en.wikipedia.org/wiki/Elo_rating_system----
My own information (this is only an example not the actual numbers used).
Lets pretend Player1 is rated 1000.
Opponent A is rated 900
and Opponent B is rated 1300.
When Player1 defeats Opponent A he will get a few points.
When Player1 loses to Opponent A he will lose more points
When Player1 defeats Opponent B he will get a lot of points.
When Player1 loses to Opponent B he will lose very little.
When it comes to draws the points will only change slightly.
The higher rated player will lose a few points in a draw.
The lower rated player will gain a few points in a draw.