How can you distinguish a wholesaler?

I have read passage "Manufacturer -> Distributor -> Dealer -> Retailer" several day before and it told me that all seller except retailer can be called wholesaler,but I'm still confused about "wholesaler",can anyone show me the exact meaning?

Well, what you hav above is true. You could say anyone between the manufacturer is a wholesaler.

Much depends on the industry / market involved. There are no automobile wholesalers, and the dealers are the retailers, so that is just two involved. In countries where the cars are not made, there may be a distributor who has the rights to import and "distribute" the cars to the dealers / retailers.

With garden hose, there may be a manufacturer, importer / distributor who brings into the country, then the distributor may have large wholesalers in major cities who will buy a truckload of hose and other products. The wholesaler may have a number of hardware stores and garden centers who are dealers / retailers and buy in order to sell to the public. The wholesaler may also sell through his own retail stores, to corner shops, even car boot and stall vendors, as well. Or, he may sell to some "cash and carry" wholesalers, where the local shops and stall vendors pick up their supply.

There just isn't one size or label that fits any of these along the way. You are asking for one label to fit where there are several layers, all of which are wholesalers.