Depending on the requirement/security/performance/UI implementation, there can be various ways to pass on data from one page to other.
Session being one of them already explained in one of the answers. You can also pass on the values using querystring, cookies, etc (various other state management techiniques that are applicable across the pages)
Adding to all that, In ASP.NET2.0 a new feature called 'PreviousPage' was added. This is something UI implementation based. Posting one page to another after a button click or so - in this scenario you can access the previous page controls and thus there values.
Refer:
Cross-Page posting for more details.[
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