Neopians may choose from over 50 different species of "animals". Most of the animals are cartoon versions of real animals (e.g. the "Nimmo" is a stylized frog), or "Pteri" is a stylized Pterodactyl. Each user can have up to four pets at one time. The pets grow older, get hungry, read books, play with toys, battle each other Pokemon-style, but never die.
In "Neopia", the imaginary land where all this takes place, there are different themed worlds: "Tyrannia" (prehistoric), "Faerieland" (fairies), "Mystery Island" (Hawaiian), "Haunted Forest" (Halloween), "Krawk Island" (pirates), "Lost Desert" (Egyptian), "Terror Mountain" (winter/Christmas), and "Virtupets Space Station". Another world, "Maraqua" (underwater), was destroyed by the Krawk Island pirates. There are also mini worlds, like Kiko Lake and Roo Island. Many worlds contains sub-worlds, for example Happy Valley and Ice Caves in Terror Mountain. Each sub-world has shops, games, and other activities. "Neopia Central" is the home-land for all Neopians and their pets. There are shops, a bank, and games to play.
Every Neopian can have a shop, "Neodeck", and "Neohome". Items can be sold in the personal shops for whatever the owner wants. These items can be bought from the official stores (in Neopia Central or other worlds), from other players, can be found randomly, or won at the auctions and trading post. The Neodecks hold a user's trading cards. Neohomes can be bought for a certain number of Neopoints for each room. Furniture can be placed in the Neohomes, and extensions (lights, insurance, etc.) can be bought.
Any Neopian can join or create a Guild, a club for users. These usually have a theme, such as the "Faerie Guild" or the "Nimmo Lovers Guild". There are message boards inside of the Guilds, only accessible to members of that guild.
Part of the reason why Neopets is so popular is that it is continually changing: new items, games, stories, shops, etc. are introduced every day. The ability to amass large amounts of Neopoints and rare items is a challenge for competitive people. The creator of the site is named Adam. Founding of Neopets? Analyzation of the idea?
"Neopians" (The citizens of neopia) earn neopoints by playing games, selling items, or enlisting help from their guild. As of December 2003 Neopia is inhabited by over 66 million users, and over 98 million pets. 
Neopets has branched out and in addition to the website, it now also produces a range of merchandise, such as plushies, stickers and hand-held video games.
Although it is commonly thought that NeoPets is owned by Adam, the creator, it was bought by Doug Dohring soon after being created.