What are the different ways to manage client states?
Client state can be managed in different ways, session state, view state or using cookies. Here is an article from ftp online: Manage Client State that discusses this in more detail.
Is there a way to let users Save data locally?
Here is an article from ftp online: Let Users Save From ASP.NET
How do we use the Wizard Control while creating forms?
Here is an article from ftp online: Let Users Save From ASP.NET
Is there a way that I can clear/expire a page cache from another page/class?
No, not easily unless you use the output cache APIs on Response.Cache and take advantage of the Response.AddCacheDependency() to make the page dependent on a common key. Then, from the other page or class you could invalidate the common key which would enforce a dependency eviction (since the key page1 depended on changed). Refer How to remove OutputCache by param?
What is a MemoryStream and how to use MemoryStream in ASP.NET
The MemoryStream class creates streams that use memory as storage instead of a disk or a network connection. MemoryStream encapsulates data stored as an unsigned byte array that is initialized upon creation of a MemoryStream object, or the array can be created as empty. The encapsulated data is directly accessible in memory. Memory streams can reduce the need for temporary buffers and files in an application. For more details refer MemoryStream Class How can I display an image from a Sql Server database?