Over 4 million jellyfish reside here, as evidenced in the episode "Jellyfish Hunter." In the episode "Born to Be Wild," it was also revealed that Jellyfish Fields is 50 miles long. There are two entrances: one at the beginning and one at the end, so it stretches out to other towns.
It is best known as a place where SpongeBob and Patrick enjoy jellyfishing, a sport in which one catches jellyfish with a net and lets them out once caught. According to the episode "The Pink Purloiner," every year, for three days, different kinds of jellyfish from all over the world come to Jellyfish Fields. This is known as "The Great Jellyfish Migration." 
The Jellyfish Fields are partially (although accidentally) destroyed by SpongeBob as he hunts down every single Jellyfish except No Name (later renamed "Friend"), so Jellyfish Fields, as it's known, was destroyed for a very short time.
Known areas of Jellyfish fields
Jellyfish Rock: The main part of Jellyfish Fields. This is where SpongeBob and Patrick go Jellyfishing. Most Jellyfish live here. This place leads to Jellyfish Caves.
Jellyfish Grotto: A cave where the discoverer of Jellyfish Fields studies about Jellyfish.
Jellyfish Coral: A coral filled with racing snails. There’s a Bait Shop and a coral that is owned by Rusty Scupper.
Jellyfish Cliff: A cliff located in Jellyfish Fields that’s full of water and has a large shipwreck.
The Brain Coral Fields: A field of brain coral on the bottom of a cliff in Jellyfish Fields. If someone replaces their brain with brain coral from the field, their IQ changes to high or above-average.
The Jellyfish Caves: The Jellyfish Caves are dark caverns in the middle of Jellyfish Fields, which is also in the foundation of Mount Spork. It is likely that early Jellyfishers lived here. The caves lead to Jellyfish Lake. This area is also where the Evil Alien Jellyfish Overlord used to stay in.
Jellyfish Lake: Jellyfish Lake is a large lake of goo, which also has a waterfall that falls down from Mount Spork. Jellyfish Lake is a peaceful place.
Spork Mountain: Mount Spork is the highest point of Jellyfish Fields, and the home to King Jellyfish. On the summit, King Jellyfish has a shower and bathtub.
Sponge-Henge: Sponge-Henge is a circle of eight large stone replicas of SpongeBob that when the wind blows through the holes, they create music, thus attracting jellyfish. The Sponge Henges were created by SpongeBob himself.
Seashell Park: A park for residents. It has several features, including a river, bridge, benches, and picnic tables.