AN ACR MLG CREW LANDED ON THE WRONG RWY. ERROR ADMITTED. MULTIPLE RWY OP; PARALLEL RWYS.

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THE LAX CTL TWR ADVISED ME THAT WE LANDED ON THE WRONG RWY AFTER WE WERE PARKED AT OUR GATE. I NEVER REALIZED UNTIL THEN THAT WE HAD DONE SOMETHING WRONG. THE SIT DEVELOPED AS WE WERE TALKING TO SOCAL APCH CTL. WE WERE ARRIVING LAX FROM THE E AND; DUE TO AN E WIND; LAX WAS LNDG AND DEPARTING RWY 7 AND RWY 6. WX WAS GOOD VFR AND VISUAL APCHS WERE IN USE. THE WIND WAS RPTED APPROX AS 130 DEGS OR 140 DEGS AT 20 KTS. ABOUT 10 MI E OF LAX; APCH ASKED US IF WE HAD A B757 IN SIGHT. WE TOLD HIM WE DID AND HE SAID SOMETHING LIKE; 'PLAN TO FOLLOW THE B757.' I ALSO THINK HE SAID; 'TO RWY 7R;' I'M NOT CERTAIN. FROM THAT POINT ON I WAS GEARED TO LAND ON RWY 7R; EVEN THOUGH A FEW CLUES TO THE CONTRARY PRESENTED THEMSELVES. FIRST; APCH POINTED US OUT TO ANOTHER ACFT AS LNDG ON RWY 7L. SECOND; WE WERE TURNED BASE VERY CLOSE BEHIND THE B757. I MENTIONED TO THE FO THAT I WAS UNSURE WHICH RWY WE WERE ASSIGNED AND TO CHK WITH LAX TWR. ON INITIAL CALL TO THE TWR; THE TWR TOLD US TO SQUARE OUR TURN TO FINAL AND CLRED TO LAND ON RWY. I THOUGHT HE SAID RWY 7R; BUT GUESS I DIDN'T PAY CLOSE ENOUGH ATTN. ANOTHER CLUE; HE SAID 'CAUTION WAKE TURB DEP B747.' WELL; A B747 WAS TAKING OFF ON RWY 7L; BUT I THOUGHT THE TWR WAS CONCERNED ABOUT OUR SPACING WITH THE LNDG B747 ON RWY 7R. THE B757 CLRED RWY 7R BEFORE WE LANDED ON RWY 7R. I SHOULD HAVE BLUNTLY ASKED THE TWR OUR RWY ASSIGNMENT.