The codes are all short for English terms. The HTML codes vormen are called tags. The extension .htm is being used less lately. But a while back it was the most used, 'cos Windows 3.x users could read this type. Now you can assume your visitors have newer Windows versions and you can use the extension .html.
Websites can become really big.
If your website will get a lot of content, make a directory on your computer especially for the files of your website. Make a subdirectory especially for images. You will notice that this makes life a lot easier when having to update things. There are several programs that give you the chance to create an HTML document in a wordprocessor environment. You type your text, place your photo and voila you can save it as .html and publish it on the Internet.