A session is a single mail-processing task. For example, a conversion from Gmail to PST with a custom filter.
A project is a collection of one or more sessions that are, usually, related to each other.
A forensic examiner may have a project called John Doe containing three sessions, one for each of the three mail stores belonging to custodian, John Doe. The first session, called John Doe - Gmail, would process Mr. Doe's Gmail account. The second, John Doe - Outlook, would process his Outlook profile and the third, John Doe - EML, his loose folder of EML files.
A school's IT technician may have a project called Faculty containing 150 sessions. Each session would process one of the 150 faculty members' Outlook profiles.
A company's IT staff might have a project, Mimecast archive, containing hundreds of sessions to convert a huge Mimecast archive of mixed employee mail.
An eDiscovery analyst may use multiple sessions in a project to categorize an individual's mail. Sessions would use the same mail source, but a different filter, to separate mail into different folders in the same PST file.
How to create projects and sessions
Please refer to the following two articles: