If you're new to the world of router bits, you need to know the tool's basic composition. Most router bits are made up of three to four simple components. All bits have a shank, a body and a tip. In the simplest of terms, the tip makes the cut, the body gives it shape, and the shank makes it all possible by connecting the bit to the router. Some bits also have a ball bearing pilot to help in guiding cuts.
The shank of a router bit slides into the collet of the router, which is then tightened to fasten the bit in place. As a rule of thumb, when inserting the shank into the collet, push it all the way in and then pull it out 1/8 " before tightening it down. This will help prevent heat transfer from the bit to your router and vice versa. Also make sure to tighten the collet as much as possible. You don't want your bit to move at all while you're using it. Some people mark their router bit after tightening it down. That way they can monitor any movement that may occur.
Shanks come in two different diameters, ¼" and ½". The ½" diameter bits will generally give smoother cuts without as much vibration. They're also less likely to bend or snap. At the end of the day, ½" bits are almost always better, but not always necessary. If you plan on sticking to the basics, ¼" shanks will serve you just fine. In fact, some smaller routers only work with ¼" shanks. However, as you get more serious and your projects get more complicated, you'll need to move up to ½" shanks and a router that supports them.
If you've been to the hardware store and realized that some router bits only come with ¼" shanks, don't worry. Routers that support ½" shanks come with adapters so they can support ¼" shanks as well. Some bits require ¼" shanks because of their small size. A ½" shank would be too big and limit your ability to make a good cut.
Now that you know the difference between ¼" and ½" diameter shanks, let's talk about router bit drill speeds.