It was a regular patty until Mr. Krabs stole growth serum from Sandy and administered it to the patty, causing it to grow.
It looks just like a normal Krabby Patty, only it is much bigger in size than other Krabby Patties. It is also covered in grease.
Role in episode
Mr. Krabs uses Sandy's growth serum on a Krabby Patty and it starts to grow. He and SpongeBob take it back to the Krusty Krab, where it eats Cashy and eventually the entire Krusty Krab. It grows so large that it covers up and eats most of the citizens ofBikini Bottom. It is defeated by SpongeBob absorbing it into his body, which makes it 'spit out' everything and everyone it had "eaten."
The Giant Krabby Patty itself could be a reference to the old B-Movie The Blob.