The ValueStack is a storage area for all application domain data that will be needed during the processing of a request. You could think of it as a piece of scratch paper where the framework does its work while solving the problems of request processing . Rather than passing the data around, Struts2 keeps it in a convenient, central location - ValueStack.
OGNL is the tool that allows us to access the data we put in that central repository. More specifically, it is an expression language that allows you to reference and manipulate the data on the ValueStack.
ValueStack and OGNL don't belong to any of the individual framework components. Both interceptors and results can use OGNL to target values on the ValueStack. IThe data in the ValueStack follows the request processing through all phases; it slices through the whole length of the framework. It can do this because it is stored in a ThreadLocal context called the ActionContext.
ActionContext contains all of the data that makes up the context in which an action occurs. This includes the ValueStack but also includes stuff the framework itself will use internally, such as the request , session, and application maps from the Servlet API. You can focus on the ActionContext as the ThreadLocal home of the ValueStack. The use of ThreadLocal makes the ActionContext, and thus the ValueStack, accessible from anywhere in the same thread of execution. Since Struts2's processing of each request occurs in a single thread, the ValueStack is available from any point the framework's handling of a request.
Typically, it is considered bad form to obtain the contents of the ActionContext yourself. The framework provides many elegant ways to interact with that data without actually touching the ActionContext, or the ValueStack, yourself. Primarily, you will use OGNL to do this.