Since NASA's iconic mission Apollo 11, which marked the first human landing on the moon in 1969, there have been six other Apollo missions to the moon. Except for Apollo 13, which had to make a quick emergency return, two astronauts landed on the moon in each of the Apollo missions 11-17. The last Apollo mission was completed in 1972 and as of 2015, no human has set foot on the moon since then. There have been however numerous probes, unmanned robots and rockets sent to the moon.
Aside from leaving behind some predictable items like lunar rovers, Apollo astronauts have in total, left 50 tons of litter. If we also consider the additional debris caused by the crashing of probes and rockets, there is on estimate 400,000 pounds (181,437 kilograms) of man-made litter on the moon. The litter and debris includes astronauts' trash like empty food packages and human waste, as well as memorabilia like a golden olive branch replica and 12 pairs of boots.
More about the moon missions:
The American astronauts were simply following a long standing American tradition of creating piles of garbage and then dumping it any old where.
They just have no respect for the Moon, the Earth, or any other planet because they are brainwashed from infancy by the wasteful and waste creating consumerist environment in which they are brought up.