The Chumporium was a temporary replacement of the Chum Bucket. They sell items and trinkets.
The owner is Plankton, who had been running the Chum Bucket for years. It's only known customer known is SpongeBob (when he bought a Snow Globe). Suspicious, Mr. Krabs confiscated SpongeBob's snow globe and threw it into his storage with Plankton's other past stuff used to get to the Krabby Patty Formula. Mr. Krabs, after throwing a party, noticed Plankton walking out of the Chumporium with a seal plush-doll, and noticed something. He looked through a window and was shocked at what he saw inside. Not convinced, he gave Plankton "A War." It may not have been a success, but Plankton had loved his gift shop and was heart broken when Mr. Krabs went on a rampage. It included lava lamps, trinkets, stuffed animals, and all other kinds of things. Plankton tried to act as if he was not trying to capture the Krabby Pattyformula but Mr. Krabs proved smarter. Plankton later tore down the sign and brought back the Chum Bucket. This transformation was only seen in the episode "New Leaf."
It is noticed that Plankton did not remove the Chum Bucket's sign. He just pasted it over with a yellow cloth with the words "Chumporium; Grand Opening." Perhaps that gave an idea that Plankton was frauding because at the end of the episode, Plankton thought he had the Secret Formula. Perhaps if he DID manage to get it, he will just get the Chum Bucket logo back on and ruin Krabs' business.