User Context
The user context is the data that is specifically assigned to an SAP user.
Use
Whenever an SAP user starts a transaction (an ABAP program), the work process processing the request always requires the user context.
Structure
Each user can open up to 16 GUI windows and therefore ABAP sessions too (
System 
New GUI Window)
.
The user context contains a user-specific area containing user and authorization data, and for each ABAP session (technical term: emode) an ABAP session context.
The following figure shows the structure of a user context.
Each ABAP session on its part can have multiple internal sessions.
Integration
All user contexts are contained in a shared resource, the SAP extended memory, which enables a fast context change.