Passing value from one page to another.

It just depends on the type and size of the data being passed.

Small information, one page to another, use querystring. Othewise use session variables.

Also, session variables will be available from any page until it is killed or expired.
QueryStrings are used to pass values from just one page to another.

As to the issue with url tampering, i generally use a hash function and append it's digest to the url to confirm against (like a checksum).  However, this was only needed in apps where a security check couldn't be done on the destination page.  Do you have an example of such secure data mrjoltcola?

Session variable is more secure way to send data across pages. As user cannot see the actual values as in case of query string. When you have large amount of data you should prefer passing it via a Session variable coz its much flexible and robust and vice versa. two ways of creating Session variables are :  

//1st way..
Session.Add("Key","Value");

//2nd way..
Session["Key"] = "Value";

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Here Key must be a string qualifier for the Session variable i.e name of the variable. And Value part is actually of object type which can store any of the two value or reference type. In the above case I have used a String type with value as "Value".

The above example assumes that you have a TextBox Control (textBox1) in your first page i.e Default.aspx the value of which you need to send to second page. i.e Default2.aspx

Hello, with the use of session variable it more helpful than the query string, Because you have to easily pass the value with the session variable.