Imagine a Walmart that sells items in bulk, which works well if you have a big family or need a lot of something (i.e. meat, paper towels, etc). They require you to get an annual membership to shop there. Compared to Walmart, Costco stands out positively because they pay good wages and don't treat their employees like shit, so customer service tends to be far more friendly. Since they only really sell things in bulk, the prices of most things tend to be fairly high even if the unit price is good, hence why other posters have said the wealthy tend to shop there (or Indians).