While the "Deals" tab is to manage active opportunities that could result in sales, the Customers tab is for recording what comes out of those deals: actual customers.
Typically when a deal reaches the "Deal Closed" stage, a button appears that, when clicked, will copy the details of that deal into the Customers tab as a new customer. It'll then add a link from the deal to the customer, in the "Existing Customer" field.
But that doesn't mean the customer won't appear in the Deals tab anymore. Even once you make a sale and your prospect becomes a customer, you may well continue to sell things to them as new opportunities arise. In this case you will just create new deals, but set the Existing Customer field to point to that customer. If you do this then you'll see all the associated deals under the Customer record, on the "Active Deals" subtab.
The Customers tab is just for customers, it's also for partners - those companies who themselves help you get more customers.
Let's take a look at the Customer record, which is quite similar in many ways to the Deals tab record:
Many of the fields are self-explanatory. You can expand the "Other Contact" area to put in the customer address and other information.
The "Alternative Names" area can be expanded to enter in other names for the company, such as DBA (doing-business-as) names, proper legal names, SPVs and so on.
Customers that you no longer deal with can have their Active flag set to No.
Next Topic: Customer Sales