In addition to Lowell's excellent explanation I would add that you need to be VERY careful about NOLOCK. Given your question I am assuming you are testing because you have heard that using NOLOCK will make your queries faster. While this is possible, you can also get duplicate and/or missing rows. This behavior is not consistent and nearly impossible to reproduce.
http://www.jasonstrate.com/2012/06/the-side-effect-of-nolock/[/url]
http://blogs.msdn.com/b/davidlean/archive/2009/04/06/sql-server-nolock-hint-other-poor-ideas.aspx
http://sqlblog.com/blogs/andrew_kelly/archive/2009/04/10/how-dirty-are-your-reads.aspx
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