app_data: a strange folder

hi,

I noticed that inVWD i can create my sql database and place it in a special folder called app_data, what is this new?
i think it is imposible because sql server is a serverdb and not a file based db like access!It's for Sql Express 2005, which is more or less a file DB. Also, the default membership provider model takes advantage of SQL Express 2005 (ASPNETDB.MDF file) automatically created under this directory.|||what are the main differences between "normal" and express edition, which is more performant?
does sql eXpress allow external connections (webs in other servers or clients)?|||It's still a server-based db, that's just it's data file. The server process will just spawn when you first try to access it.