One thing I have noticed about nearly all the nodes and write ups about Neopets, is that no one has yet mentioned the "shop" or the "Stock Market" features.
Neopets, while based primarily on the raising of ones pets, is also about making Neopoints, or NP, as has been said. The shop feature allows the owner to sell items that has no use to the them, or so that he or she can make a nice little profit, therefore, raising the amount of neopoints they have.
I myself, have a shop and use it to generate a nice profit.
This could quite possibly be why neopets is so successful and popular. One isn't forced to play games to earn "money". You can use your skills as a buyer or seller to make profits. It definitely adds to the appeal.

The stock market feature is also a profit making venture for any owner, although quite often playing the stock market results in the loss of neopoints if one isn't constantly in the neopetss site.
Based on "real life" Stock markets, one buys a certain amount of shares (limited to 1000 per day) at a low price then sells them if they go higher.
Unfortunately, as I said, if one isn’t in the site at the right times, the particular stocks that have been bought often plummet in value, causing the loss of quite a lot of neopoints.