This seems to be one of those immortal questions, its been answered in full by lichess staff members several times now you might have some luck finding a suitable answer by running a forum search.
One of the main differences to note is that lichess uses "glicko" in place of the more familiar elo system.
I also notice in some cases it differs depending on the players rank. For example some titled players have the same rating on lichess as chess.com. it seems to even out some after so many thousand games (30,000/40,000 or so). It takes some looking into to fully understand it again I would suggest some forum searches.
One of the main differences to note is that lichess uses "glicko" in place of the more familiar elo system.
I also notice in some cases it differs depending on the players rank. For example some titled players have the same rating on lichess as chess.com. it seems to even out some after so many thousand games (30,000/40,000 or so). It takes some looking into to fully understand it again I would suggest some forum searches.