A Schema is similar to a class in most object-oriented languages, or to a C struct, while an Entity is an object of that class. The Schema object is immutable. To create and populate a Schema, use the SchemaBuilder class. Schemas are stored in the memory of the running instance of Revit and may be retrieved with the Lookup method. When a document containing Entities of a Schema is saved, the Schema is saved with the document too. Opening that document reintroduces the Schema into memory. As you plan to store your data in Revit, please be aware that the data will be stored in Revit elements. Overwhelming the Revit database will impact performance and stability, and will make the user unhappy. Also, remember that multiple add-ins may be storing their data simultaneously. Several kB per element or several MB per file are reasonable maximums for one add-in. If you have larger requirements, consider storing them in a separate database (such as SQLite) and storing keys into that database in Revit.

System Object
Autodesk.Revit.DB.ExtensibleStorage Schema