An estimate (sometimes called a quote or proposal) shows the customer what something will likely cost. It usually has an expiration date and is not yet a request for money.
An invoice is the bill. It includes a unique invoice number, due date, payment terms, and a total the customer owes. Once issued, it's a record for both your books and the customer's.
In Glo Invoice, you can convert any approved estimate into an invoice with one tap — line items, taxes, and customer info carry over automatically.