A can opener (also known as a tin opener) is a device used to open steel (not tin) cans.
Simple can openers, like those found in pocket knives, are operated by walking the device around the edge of the can while digging into the lid. The lid will have very jagged edges after removal.
Most can openers press a cutting wheel into the edge of the lid while a rough support wheel presses up under the lip of the can. It is best to purchase a can opener that drives this support wheel to provide easier motion. Non-electric can openers with this feature can be identified by a set of gears behind the cutting and support wheels. The pictured can opener has such gears, with 2 or 3 teeth visible peeking out above the cutting wheel near the bottom of the picture.
Some more unusual can openers cut into the side of the can, often within the seam area so that a nice lid is formed. The lid is not very sharp and does not fall into the food. In this case, the support (and possibly drive) wheel runs around the inside edge of the lid as the can is opened.
Be sure to clean your can opener after use. Give it a rinse, then run a paper towel through it. In case you or someone else has been careless, you might also clean the can opener before use.