They're just different writing standards applied to different types of writing. When you decide what style of writing you plan to do, find out what the standard is and follow it. For something like casual blog posts, you don't need to worry about them though. Some publications actually have their own official writing styles as well, so the ones you mentioned aren't quite a be all and end all.
Look it up. Search for each, and you'll find out what they're each commonly used for. It's not hard. Just Google "MLS style," "APA style," etc.