Without getting too technical, WMS stands for Warehouse Management System. Believe it or not there are still fulfillment companies who use physical paper or excel worksheets to try and keep track of your inventory.
Would you trust your savings account to a financial institution that manually tracked your money?
Now the majority of fulfillment companies should have software that manages the movement and location of your inventory through the warehouse at any given time. But most of these WMS systems will be either developed in-house, or for less sophisticated and smaller fulfillment companies they may have purchased a very entry level WMS.
Here's a quick tip: What do uber successful brands like Hautelook, Overstock, Gilt Groupe, ProFlowers, Wayfair, and Moulton Logistics all have in common? We all use the same enterprise-grade WMS-- Rated Tier 1 by Gartner's Magic Quadrant. So although it's become standard jargon for fulfillment companies to call themselves, "leaders in the industry," we have the systems to actually back up that statement.