Categories of User Assistance
By its very form, the phrase user assistance implies the existence of a tool, i.e., the thing used. The existence of a tool implies an application or goal, i.e., what the user is trying to accomplish with the tool. In other words, the existence or a Word Processor implies the user's need to create a document. So user assistance is help in using a tool to achieve a goal. In this sense, user assistance can be divided into two main categories: (1) How to use the tool and (2) How to use the tool to...
The first is very tool-centric and focuses on the rules and manipulations of the application itself. Good user-experience design minimizes the extent to which this kind of UA is needed by making these manipulations and interactions self-evident, but some degree of this assistance will be required in most applications. How to enter a summation formula in Excel would be an example of How to use the tool user assistance.
The second category of UA, How to use the tool to..., is more user-centric and focuses on goals and problems within the user's context. Showing someone how to use Excel to do a budget would be an example of How to use the tool to... kind of UA.

Levels of User Assistance
At its most basic level, UA is discourse that explains something and can be delivered through channels that are more or less detached from the tool, e.g., manuals or compiled help files. As it becomes more advanced, it becomes more interactive and integrated with the tool. Embedded help and bubble help are examples of more highly integrated user assistance patterns. At a more advanced level, user assistance acts like a Performance Support System and is highly integrated within the tool, e.g., a Wizard. At its most advanced level, it becomes the tool. For example, is spell-check a feature within a word processor or is it user assistance?