Here is what one website has to say:

About 1885, merchants all over the United States started using a form of advertising called a trade token (or chit, or bingle, or "good for"). When a customer bought something, they were given a token that resembled a coin. On the obverse, the token gave such information as the name of the business, address, town, state, etc. On the reverse, something like "good for 5¢ in trade" or "good for 1 cigar" were often used. When the customer returned to the store, they were given credit or their purchase was discounted by the amount stated on the token.

The sizes, shapes and materials that were used to make trade tokens varied widely. The sizes ranged from smaller than a dime to larger than a silver dollar. Common shapes were round, square, scalloped, oval and rectangular. (The majority were round.) Although most tokens were bronze or aluminum, other materials such as paper, fiber, zinc, copper and bi-metals (bronze and aluminum) were also used.

The merchandise that the token was "good for" would make an interesting collection of its own. Some of the common "good fors" were in trade, in merchandise, in cash, a cigar, a pint or a quart of milk, a tune, a shave, and a drink. Many others such as 1 box of peaches, one card game, 1 pack of cranberries, one manicure and one loaf of bread are known.

Some collectors have assembled interesting collections of the different denominations represented on tokens. The most common would be "good for 5¢ in trade," with 2½¢, 6¼¢, 10¢, 12½¢, 25¢, 50¢ and $1.00 also reasonably common and easily found. Although many other denominations exist, they are harder to come by and often command a premium.

Most collectors have one or more particular topic they collect. Probably one of the first that many collectors choose is tokens from the town where they live.

http://www.pnna.org/reference/trade_tokens.html