DISC (uppercase "i") refers to the original behavioral model created by William Marston.
DiSC (lowercase "i") refers to a specific implementation of DiSC, currently published by Wiley.
Everything DiSC is the latest generation of DiSC assessments, also published by Wiley. This version of DiSC has several applications (Workplace, Management, Agile EQ, Productive Conflict, Sales, and Work of Leaders) available in the traditional PDF form, and introduces the Catalyst platform. Catalyst is web-based and can deliver results for Workplace, Management, and Agile EQ but also has interactive functions for coworker comparisons and for exploring group dynamics.