Displaying posts from only one category

http://wpgarage.com/code-snippets/display-posts-from-specific-categories-on-a-page/

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http://codex.wordpress.org/Function_Reference/query_posts

Show Posts for One Category

Display posts that have this category (and any children of that category), using category id:

query_posts( ‘cat=4’ );

Display posts that have this category (and any children of that category), using category slug:

query_posts( ‘category_name=staff’ );