A router is a high speed cutter that uses special router bits. These bits have different profiles to make specific type cuts or slots. One will cut just a square slot that can be used for a shelf support. Some bits have ball bearings to run smoothly. These ball bearing router bits are worth the extra cost.
Some of the bits have profiles to cut the edge of a board. This provides a nice rounded edge to your board.
Routers can be dangerous. Be sure to unplug the power tool before changing bits. Use two crescent wrenches to change the bit. Use all the necessary safety equipment, such as goggles. Read the manufacturer's instructions and warnings. Handled properly, a router is a great tool.