Defining which projects are leads and which projects are jobs is actually up to you! You can follow these guidelines:
A lead is a project that is a potential booking. Leads are often new inquiries or projects where you've sent a proposal but don't have a signed deposit or paid invoice yet.
A job is a for sure booking, however you define that! For many businesses, a job has a signed contract and at least a deposit paid on an invoice.
The idea is that new projects will start off with a lead status and eventually move over to a job status once they are booked. You can create custom project statuses under the categories of leads and jobs to more closely track the milestones in your process.