The Grand Slam is a tennis tournament, while the ATP is an orginization. (Association of Tennis Professionals.)
The tennis Grand Slam is a annual tournament. There three tournaments within the Grand Slam. The US Open, the Wimbledon, and the Australian Open.
Wimbledon Grand Slam Tournament is played on Grass Court.
Technically the ATP season ends some time after the U.S. Open so it is the last Grand Slam Tournament played, but in the next season the Australian Open is the next Grand Slam tournament played after the US Open.
The US Open is the ony Grand Slam tournament that does not use tie breakers in the final set.
The Australian Open