The presentation context is part of the DICOM negotiation items and consists of the Abstract Syntax (what information is exchanged) and Transfer Syntax (how it is encoded).

The Transfer Syntax specifies the Byte ordering and whether or not the VR is specified as part of the DICOM message exchange (Implicit or Explicit VR) and compression. Value Representations (VR) are format or type definitions for DICOM elements (Attributes).  The Transfer Syntax defines how information is exchanged and how data is represented.  

The presentation context is always the combination of a SOP Class and one or more Transfer Syntaxes.