Tests for a specific category-ID using the in_category function. If the category returns the appropriate test result, a different CSS class will be displayed. From Codex: Style_Posts_From_Some_Category_Differently
Tests for a specific category-ID using the in_category function. If it is true, The Loop will skip processing that post and go to the next one. From Codex: Exclude_Posts_From_Some_Category
You can style a category of posts to look like a separate blog by creating multiple Loops. From Codex: Multiple_Loops