L is a lens grading created by Canon and are specific to canon lenses. Some people reckon the "L" denotes "luxury" (more likely correct) but I'm personally partly more inclined to go with "Low dispersion" for the sake of diplomacy, but then again if that were entirely true the entire time L lenses have existed why did they not call them LD lenses to begin with? It's like someone just makes this stuff up as they go along whilst deciding to say it mean "Low dispersion" in order to sound like it was done on technical grounds and to come across as less elitist in trying to make people not think it originally meant "luxury" because of the original marketing ploy that was used to try and sell them to people with the money to buy them. (...and breathe...) Anyway back to the point, the L lenses are the upper pro grade lenses that command a high premium price as a stand alone after sales item. They generally don't come as standard with camera bodies or camera body kits and are usually sold as individual items (more often than not costing more than the camera bodies themselves!). They posses far superior optics and build quality than standard kit lenses. L lenses can help the user capture sharper images with more vivid colours just through their physical optical properties alone even before any kind of digital editing might be applied with the appropriate expertise. Once you've used an L lens you'll find it difficult to go back to using standard kit lenses if pro grade optics for producing the best image quality possible is your main concern. They also generally tend to retain their value much better than standard kit lenses when sold as second hand. 

Prime lenses usually have a fixed zoom whilst allowing you to make adjustments to the focal depth. They also tend to have the kinds of optical properties that allow for more pristine images that are less tainted by optical flaws. Many generally have a larger potential maximum aperture which allows for greater light transmission and faster camera shutter speeds. This in turn allows for many of them to handle much better in lower lit conditions which result in less potential blur (with the appropriate camera settings) whilst capturing movement. The wider the aperture the faster the lens is (usually with suitably matched optics). What we mean by a fast lens is that it is able to capture movement and motion much more quickly with the potential for less blur (dependent on lens optical properties). A wider aperture does not always result in sharper images which is why certain lenses need to be stopped down a little. (stopping down means to decrease the aperture size to a smaller standard setting). However there are lenses out there that have a constant open aperture of different ratings that cannot be adjusted/altered, but in such cases the optics have been designed and implemented in such a way that the lens will enable you to capture sharp images even with a fixed aperture that cannot be stopped down.