As strange as it may sound, form plugins are those jQuery plugins that, in one way or in another, are related to forms, be it for spicing up forms a little (changing colors, adding pictures, or whatever) or for validation purposed styling and/or usability improvements.
After all, forms are a practical and functional way of submitting any kind of information in different ways, but often lack security and give spammers and really annoying people the n-th chance to eventually spam and annoy more.
As for many other jQuery plugin categories, a lot of people have given their contribution in terms of code, functions, and plugins. Many interesting solutions have been proposed to solve some of the most common and widespread problems that arise when developers or users need to use forms.
For one, there's the always present problem of validation. Be it due to misunderstanding, distraction, or even on purpose, people are likely to submit forms containing missing or wrong information...