A distributor is HUGE. They carry fabrics from a large number of manufacturers, and usually notions, patterns, tools etc. They buy below wholesale and sell at wholesale prices. (I've always wondered how much they get to mark up their prices but have never found out!)
Anyway... they typically have a few favorite lines that they keep in stock (including basics, such as their favorite line of solids and blenders) and the rest they special-order from the manufacturers, as they receive orders for those items from their own customers. It can take 6-8 weeks for them to receive the goods and send them out, if you're ordering something that they do not have in stock.
A business customer is basically anybody who can buy in bulk - full bolt(s) - who also has a tax number (this is the number that a state issues, for the purpose of authorizing a businees to collect sales tax).
If you're just a regular person who wants one or two full bolts to do a big quilt/decorating project, then you won't have a tax id and the manufacturers/distributors will not sell to you. If you're a "professional crafter/quilter/sewer" then you probably have a tax id (required for most of the larger arts & craft shows these days, at least in NY).