The Holo fuel app, account, and wallet are all the same thing.
The Holo fuel app functions as your Holo fuel account where your transaction history and Holo fuel balance will be stored. One day, it’s possible that a hardware wallet could support Holo fuel.
Each user will have their own identity, account (‘wallet’), and source chain on Holo; giving you the ability to transact with others and develop a log of transactions recorded to your source chain. The sum of your inbound and outbound transactions in your transaction history determines your current balance.