Lead = potential client who is interested but has not signed yet. Lives in CRM → Pipeline.
Deal = synonym for lead with a concrete value and stage in the funnel. In Senly we call them "leads" (the counterparty is called "contact"); in other CRMs like HubSpot they are called "deals". Functionally the same.
Client = a lead that has signed (stage = Won) and has been converted manually or automatically into a client profile. Clients live on the Clients page, with their own tabs for content, tasks, finance, etc.
Normal flow: contact → lead/deal → client.