Here are a couple of the finer points to take into account when you’re shopping for router bits. In certain situations you can improve the quality of the cut - and the quality of your experience in making it - by using either a shear or a spiral bit.
How are they different? Instead of running parallel to the body and shank of the bit, the flutes of shear and spiral router bits are angled - a simple difference that can have considerable impact on the way the bit performs. In general, shear and spiral bits cut the surface of the stock with a slicing motion instead of chopping straight into the stock. The result is less energy consumption, less vibration and a cleaner cut.
Shear and spiral bits behave differently depending on whether they have an “upcut” or a “downcut” configuration. The down-shear of the flush trim bit you see above forces chips and shavings downward away from the router, and also has a tendency to pull the base of the router downward toward the material. Both of these behaviors turn out to be "features” when you're using a handheld router to make flush cuts along an pattern or straightedge: The first keeps the work surface clear of debris, and the second makes it easier to keep the base of the router flat on the surface of the workpiece. A downcut bit also helps limit splintering on the surface of the material.
The upcut profile on the spiral bit you see on the left, on the other hand, propels chips and shavings upward out of the cut, making it the bit of choice for mortise work. A good quality spiral bit makes smooth, fast work of cutting a deep mortise, and the upcut orientation keeps the mortise from getting packed with shavings.