'Camouflage' is any coloration or decoration that allows a geocache to blend into its surroundings. Camouflage helps make it less likely that the cache will be muggled, and also makes it harder for geocachers to find the cache.
Examples of Camouflaged Caches
Hunting and sporting goods stores carry a variety of camouflage tape patterns. The material is very much like gaffer tape. Camo tape is easily applied, but it can sometimes peel and doesn't stick to itself well. Prices vary.
Many non-camouflaged containers can be painted. Spray paint is the easiest to apply. Solid colors work well in many situations. Multiple applications of different colors works well. Textured faux granite spray paint is more expensive, but it can camouflage a container very well. Round containers with granite paint can be nearly impossible to tell from actual stones.