Crystal Lake is a small town in the state of New Jersey. It's population in 1979 (the events of the original murders by Pamela Voorhees) was only 4,350. It is also the location of the imfamous location of the "Camp Blood" murders, Camp Crystal Lake.
It is often thought by a majority of Friday the 13th fans that Crystal Lake and it's adjacant summer camp property, Camp Crystal Lake is located in New Jersey. But the exact location within New Jersey is unknown but is apparently a very rural area - possibly within the Pine Barrens.