Content
The Design model generally consists of UML diagrams plus narratives where necessary. The model describes the static structure and dynamic behavior of each class. Additional non-behavioral requirements must also be included in the design document.
In the 4+1 view, the design is the details of how to implement the logical view and functional view. It is any additional information to fill out the component view, and the plans for the final implementation. The information added to each view is technical, not business-oriented, and is an elaboration on the initial design decisions made as part of Architecture.
Process
The process of design refines the responsibility assignment begun by the architecture effort. This involves consideration of algorithm and data structure. It identifies opportunities for reuse via inheritance. It applies design patterns to create robust, reliable, reusable software.
Note that the modeling process seeks to create more refined and formal models of the problem universe. The design document is the next-to-final level of formality. The final level is the source code for the application software.
Standards
The level of detail varies with the audience. Experienced developers may be able to proceed with a high-level class diagram. Novice developers may require sequence diagrams for more complex methods.