Sql-server – the difference between SQL Server standard and web edition

This should answer your question:

MSDN: Downgrading from SQL Server 2008 R2 Enterprise Edition (trial) to R2 Web Edition (licensed)

You only need to uninstall Analysis service before running edition upgrade. And, of course, make a just-in-case database backup.

Standard has more roles, like Domain Controller, File Server and all sorts of other enterprise roles. Web has only Web Server, DNS etc, which is good for web hosting. Web is much cheaper, because is designed for scale-up, were you dont need a CAL for each visitor accessing the server via internet, while for the File Server in standard, you pay for each domain user or domain PC.