You’ve asked a lot of questions, and I can understand your frustration. Let me suggest that you view these videos and article here on Videomaker that explain what codecs and file formats are all about. Note that some, like the first link, are a little out of date, as Youtube no longer used Flash for video, but the explanations are still valid:
Web Video
http://www.videomaker.com/video/watch/tips-and-techniques/514-web-video-compression-and-codecs
Video File Formats:
http://www.videomaker.com/article/c10/15362-video-formats-explained
There’s a lot more using this search:
http://www.videomaker.com/search/google/codecs?query=codecs&cx=016301996033947342722%3A-xsmlwign8w&cof=FORID%3A11&sitesearch=
After reviewing the articles and videos, come back with more specific questions.
