The word fetish probably comes from Latin facticius (artificial), by way of Portuguese feitico and French fétiche. A fetish can be any object. The important point is that the person who carries it must either have a strong emotional connection to the object or regard it as representing a higher authority (such as a nature spirit or the Divine).
The cross of the Christian church is, by all definitions, a fetish. It represents a higher power (Jesus), and many people wear small crosses as items of protection and blessing. The rosary comes under this heading as well, representing the Virgin Mary, to whom prayers are directed for assistance.
These conditions distinguish a fetish from an amulet or talisman. In some cases, though, you'll find some overlap. For example, a pagan police officer might use his badge as a fetish because it represents a power (albeit mundane) and because he has it with him constantly.
A fetish represents only one objective — you wouldn't carry a single fetish for love, protection, and success. Many fetishes serve as “one shot” magick spells — you need a different one for each instance when help is sought. A witch might make up a bunch of fetishes at the same time, all of them designed for the same purpose. For example, she might fabricate a number of fetishes to stimulate artistic endeavors, wrapping bay leaves (to represent the sun god Apollo) in pieces of yellow cloth (the color of creativity) and empowering those bundles with an incantation. Then whenever she hits a dry spell and needs a little inspiration, she can use one of the bundles.
You activate the energy of single-use fetishes by carrying them, burning them, burying them, or floating them on moving water. Burning releases your wishes to the heavens and disperses the energy. Burying helps the energy grow. Floating in water helps transport the energy where it's desired.