Almost anything can be used as a charm, but like amulets they generally take a form representative of their intent. Charms can be thought of as devices for attraction - So a heart might be used to attract love, a horse shoe, good luck, Thors Hammer, power and protection, a Dove, peace and so on. It's what an object symbolises to the wearer which is important.
Charms are generally quite small, often unobtrusive and can be worn as a pendant or on the wrist as a bracelet, around the ankle, suspended from a belt around the waist, or even carried on a handbag, purse or a mobile phone - something that will be with you most of the time.