Leads (Clients → Leads) are prospects who haven't become customers yet — including anyone who fills in your public enquiry form. Convert a lead to a customer when you win the work.
Customers (Clients → Customers) hold everything about a client: their sites, contacts, job history, quotes and invoices. A customer can have many sites (e.g. several pitches or grounds), and each site has its own address that drives mapping and scheduling.
To add a customer: Clients → Customers → Add customer, then add their site(s) and contact details. You can also bulk-import existing customers from a CSV.