Fill the crack with urethane caulk. Snip the opening of the tube at a 30-degree angle, making the opening the same size as your gap. Use a smooth, even motion, filling the crack flush with the surface, beveling it if it's against the house. Smooth the caulk in wide joints with the back of an old spoon. Wipe the spoon clean as needed with a rag and mineral spirits.
Cracks and gaps in concrete are more than just an eyesore. Water can get into the joints, freeze and then expand, making the cracks even larger. Gaps against a house can direct water against the foundation, leading to more problems. Once a year, go around your home and fill these gaps and joints with urethane caulk to prevent problems. The caulk is available at contractor supply stores, well-stocked home centers and hardware stores. For gaps and joints more than 1/4 in. wide, install foam backer rod to support the caulk. You want the rod to fit tight in the joint, so buy it one size larger than the gap.
A word of advice: Keep the urethane caulk off your bare hands and clothes; it's the stickiest stuff you'll ever touch. Wear disposable gloves when you're tooling the joints. If you get some on your skin, quickly wipe it off with a paint thinner–dampened cloth.
Skip the backer rod and apply caulk directly to cracks narrower than 1/4-in.