Posted by I always seem to keep forgetting when I backup from one server and restore to a new server that SQL Server 2005 will recreate the users from the original server, and they become ‘orphaned’ as they don’t match to the similarly named user on my system automatically, and thus cause an error when an ASP.Net application tries to use its web.config settings to access the database.
This Microsoft Support article gives the steps needed to fix this orphaned user issue as well as create it in the first place, so this is a cool resource to have when training others on the issues they may face and how to mitigate them when wearing their database administrator hat.
Redditech 'Ground Zero' Blog 
Yeah, I think this is easily one of the areas where many SQL Server users run into trouble too. As such, I created a video that covers what’s going on to cause this problem, and which also shows step by step instructions on how to rectify it:
Feel free to check it and other free videos out on http://www.sqlservervideos.com.
Woops, sorry… here’s that url again (wasn’t sure if the comments required a different/custom approach to tags, or just actual tags):
Video: Copying and Moving SQL Server Logins.
It’s very simple to find out any matter on web as compared to books, as I found this piece of writing at this web page.