in 
web.config file
<appsetting>
<add key="constr" value="data source=database name;
                      database=database name"/>
</appsetting>

and 
 in .aspx file
string str=configurationsetting.appsetting.constr;

In web.config file just plase

<appSettings>
<add key="ConStr" value="Provider=SQLOLEDB.1;User 
ID=sa;Initial Catalog=Sample;Data Source=ABC"></add>
</appsettings>

and in CS file

sqlconnection con = new sqlconnection
(ConfigurationSettings.AppSettings["ConStr"]);

and u can u this connection any where in ur App.