what is the difference between a Customer and an Account?

Account and Customer both are different things. 

Customer lookup tab is used to manage your guest data. from the eZee Optimus front end, you can Add and edit the guest record. and from Guest Database tab available in the eZee Optimus back under Master tab you can add, edit, active/deactivate or remove data from the guest database. 

At the time of taking the order, we can use that guest detail in eZee Optimus Front.

On the other hand, Account lookup tab available at the front is to manage to credit accounts. Many time restaurant sign contracts with the corporate company and have an account created which will pay for their employees when they use the restaurant’s services. From the front, we can add payment, in particular, city ledger account or void payment. 

This account offers a credit limit with a time frame (e.g. Week, monthly) to clear credit. The dues are posted to the account whenever an employee of that company uses the services of the restaurant. The company can then take care of the due amount by the agreed time frame. 

Basically, these are credit sales. So you can create a credit account for guest as well. You can generate credit report as well for same guest.