There are many types of caulk. The one you choose will depend on your project. If you will be painting over the caulk you cannot use a silicone-based caulk since it is not paintable. Silicone will bond to almost anything, however, making it a good sealant for non-porous substances such as ceramic tile, glass and metal surfaces. A masonry caulk retains elasticity and will remain in cracks in mortar and concrete.
Caulk comes in many colors to match the existing surface color.
You can even buy a clear caulk. Caulk is applied from either a squeeze tube or a caulk gun, with the gun usually being the preferable of the two in laying a precise bead of caulk. To begin, slice open the tube of caulk at a 45-degree angle with a utility knife. Practice a little with the gun, engaging and releasing the trigger. After laying down the desired bead of caulk, immediately release the trigger which will halt the flow of caulk.