Router bits come in two varieties: ones that form edges and ones that make grooves. Many edge-forming bits have a roller bearing that keeps the bit lined up with the edge of a board or template as the cut is made. When using groove-forming bits, an edge guide or clamped straightedge keeps the base in line as you push the router over the work. Without these bearings or guides, a router will wander, no matter how hard you try to keep it straight.
Bits can get pricey—some big ones go for about $50 a piece—but a good basic lineup, like the ones above, can be had for about $100 or less. Look for carbide-tipped bits, which last longer and stay sharper than ones made of high-speed steel. Bits with ¼-inch shanks cost less and can handle small work, but a good ½-inch bit is stronger and generally makes a better investment, if your router’s collet is big enough to handle it.