what is diffrence btween sql server 2008 and sql server 2008 express?

Express version has very limited features specially server side and enterprise/reporting. Also, Express version allows only one connection at a time. That means, you can't really use a SQL Server Express database in Web application that expect more than one users.

Express version is just for learning purposes or use it on a single machine with Windows applications that needs small data repository on local machine.

Here are some comparisons:
http://www.microsoft.com/Sqlserver/2005/en/us/compare-features.aspx