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Copper bands, tape and foil is thought to work by reacting with the slime that the slug or snail secretes producing an mild electric current. The smarter mollusks will turn back at this point, should it choose to continue the irritation stimulates over-production of mucous,  causing them to dry out. Available in thick copper bands or as thin adhesive foil it is used to form a circular barrier either around the plant stem in the case of the bands or stuck around the side of the pot. 

To act as an effective barrier the recommended width of the band should be 5cm (2inches). Slugs and snails have rather small brains and it takes them a while to get the message that something bad is happening, reducing the width of the band may allow the slug to cross it before it's even realized what's going on. Once on the inside of the perimeter it's more likely to stay there.

An advantage of using copper bands is that they look good and can last forever as long as they are cleaned with a mild vinegar solution to remove the brown patina which forms on the surface over time. Initially they are a little expensive (but remember they last a lifetime) so if you're considering using a lot of these it would be cheaper to buy a large copper sheet and have it cut into strips so you can form your own rings. The tape and foil will need to be replaced at intervals, being thinner it is much more likely to get damaged through general wear and tear.

Another advantage is that they don't kill the slugs and snails making them available to natural predators.