A deal can have several statuses:
All deals that have come in or have been manually created, but haven't been replied to are automatically marked as new.
This label is reserved for deals that have received at least one reply from you, and are in general state of conversation about the deal.
Once you send a proposal, the status changes to "proposal sent" to indicate that you are waiting for a reply to the proposal. Once you receive the reply, the deal is labeled with "negotiation" again.
Won, or Lost
You can mark deals manually as "won" or "lost" yourself, and additionally you can revert them to the stage of "negotiation" as well. Marking deals with "won" and "lost" labels is super-useful for keeping track of how many deals you have won, vs. how many you have lost.
Together these statuses form what the professional sales people call a “sales funnel”. Your deals go through each of these phases in order to be marked won or lost.