Negative photoresists have a chemical structure that allows the areas that are exposed to light to develop at a slower rate than those areas not exposed to light. Example: In theory, isolated lines or islands are best printed in negative photoresist, whereas spaces and contacts prefer a positive resist. With negative resists, exposure to UV light causes the chemical structure of the photoresist to crosslink/polymerize, which is just the opposite of positive photoresists. Instead of becoming more soluble, negative photoresists become extremely difficult to dissolve. As a result, the UV exposed negative resist remains on the surface while the photoresist developer solution works to remove the areas that are unexposed. This leaves a mask that consists of an inverse pattern of the original, which is applied on the wafer (or other substrate).
References: and http://www.lithoguru.com/scientist/glossary/index.html