A family of Microsoft relational database management systems designed for ease of use.
To add to what ScottG said:
Even with inventory you own, you still must add it to your inventory. This servers as your baseline inventory (your starting point), and you then add and subtract to this. If you look at the Inventory Transactions table in Northwinds, you'll see a Transaction Type column. The "Purchased" type would be incoming inventory (of one sort), and that's how you'd add your current inventory to your new system.
If you look at the design of the Inventory Query, you'll see all the items needed to properly determine inventory counts in Northwinds. Note this is a somewhat primitive inventory system. Most have quite a few more working parts, like Returned items, short shipped items, etc etc.