Good to hear that you want to clear your concepts. Let me explain with an example.
Suppose you have created an account in "ABC" bank. Bank requires all the details like Name, Address, email, age, account number, profession, etc. All these details need to be stored somewhere. Here database comes in picture.It saves all these details in the database.
Let's say you have called its call center for an enquiry. Application needs your details like accountid, your name, etc.It retrieves all these details from the database. Database operations are costly & time consuming.
You may ask request for different assistance during the call like credit card payment information.So instead of calling the database again & again, pega pre-loads the data in memory.
Here data page comes in picture.You can use D_Customer,D_AccountDetails to preload the data in clipboard and use it whenever necessary.
When we refer the datapage in any rule,the data page gets loaded in the clipboard. On subsequent access, the data page never gets loaded since it can use the same page in the clipboard.
It stores data locally that's why It can be referred as local storage.