UML (unified modeling language)
is a way to depict code using a standard set of objects/notations that can be quickly understood by other developers. UML or UML-like designs can often be generated from existing code in VisualStudio or through other programs.
HLD - High level design doc
example
This doc will talk about the design at the highest level. What services/programs are running on what machines/data centers, how these data centers interact, what other systems are involved (sql machines, firewalls, web sites, etc). This can be thought of as an architectural design doc as well.
LLD - Low level design doc
example
This is going to be highly technical look at your design. You will include your UML diagrams here, split up and explained with pseudocode if possible. It will also include message schemas, XSDs, and any other technical information that you can gather.
Both the HLD and LLD generally have a pretty standard layout to help get your information together.