Are unearned revenues earned but not yet billed?

No, Unearned Revenue is revenue that the person/company has received from the customer but has not yet fulfilled the commitment that they are obligated to fulfill. A better example.

Let's say you are a computer company and your customer orders a $1500 computer. The customer pays you for the computer but you haven't shipped the computer to the customer yet. The $1500 you received from the customer is unearned revenue. Unearned revenue is recorded as a liability until the obligation owed by your company has been fulfilled. This is because, even though your company has received the money for the order they have not fulfilled it and are liable to the customer to either fulfill the order as promised or if unable to do that, refund the customers money.

The entries above would be something like....

Cash Debit $1500

Unearned Revenue Credit $1500

Once the order is fulfilled and the customer has been shipped the computer and adjusting entry would then be made to reflect that the revenue has been earned something like:

Unearned Revenue Debit $1500

Revenue Credit $1500

This basically just moves the amount from the unearned revenue account to show that it has been earned. Cash had already been received so no adjusting entries would be required to the cash account.

Revenues earned but not yet billed would be an account receivable. If the customer gets the computer and hasn't paid for it yet, you've earned the revenue that would come from the computer but you haven't received the money yet. At this point the customer owes you (the company) and accounts receivable is debited with the amount owed.