An 'Estimate' can be an offer that you make to a client or a prospective client. It could be a simple sales quote. Any business proposal, really, for the purchase or a service.
You can use the "Estimate" functionality to send your clients those offers, allowing them to accept estimates directly through Elorus's client portal. If you wish to simply use their email, you have the tools to manually mark acceptance (or rejection), as well.
Most importantly, you can use the functionality to create invoices, with couple of clicks. Details will pass from the estimate to the invoice so you can forward to your client seamlessly.