Most retailers discuss store traffic as they hope that the product selection they offer brings customers back again and again. At Costco Wholesale (NASDAQ: COST), that's not the most important number. The key metric is membership. And right behind that is another number that would seem to make more sense for a magazine than for a store.

Here's what you need to know to understand just how successful Costco really is as a business. Let's dive in.

Costco charges a fee for using its store, making it what is often called a club store. There are basically two different levels of fees, with one level of membership for the average person and businesses and a higher-cost membership for "executive" members. (The key feature of the executive membership is a 2% cash-back reward.)

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Source Fool.com