You must ask such questions under the article or blog post itself, it helps the authors to maintain their article contents to help the readers and can also help any other reader if he has the same question. Asking a question on a separate forum doesn't help you, readers or anyone.
ASP.NET Identity is a full interface based authentication provider, there are many services, stores, managers in it which you can use for your own needs. Entity Framework then comes in handy to handle the data sources for your application. ASP.NET no longer only requires SQL Server for data source, you can use any data source. For that sake, Entity Framework is used to manage the data sources.
The
UserStore[
^] is a component that is well described by MSDN as,
Quote:
Represents an Entity Framework implementation of a user store that supports IUserStore, IUserLoginStore, IUserClaimStore and IUserRoleStore.
You would need to learn the Identity framework, there are many things that cannot be explained in a compact answer. You should start learning from MSDN,
here[
^].
Edit
As the name suggests (and what I said in the previous content) the UserStore is actually the data source from where your user details and records would be fetched. You can define your own data sources. The class actually is present in Entity Framework. In your own application you implement the IUserStore interface and add additional features for your own application.
For more in-depth discussion and overview of this object in ASP.NET, please read the ASP.NET guide at
http://www.asp.net/identity/overview/extensibility/overview-of-custom-storage-providers-for-aspnet-identity#userstore[
^].