HTML is a computer language that allows you to put information into a document on the internet that when it is called upon by a web browser (or other software) is viewed by the viewer.
HTML code is comprised from standards set by the W3C or World Wide Web Consortium. The W3C meet from time to time and make changes to the standards, add new standards so that websites and browsers are able all view the site the same way. BUT DON’T LET THAT FOOL YOU. Just because your site is W3C Compliant doesn’t mean it is going to show up how you want it to look across all browsers and all computers.
An example is if you look at most any Static Page website like Coffee -n- Caffeine you will see it looks one way with Internet Explorer and slightly different with Netscape or Firefox or Google Chrome. This is expected. Each browser handles the code that it is given by the web page a little differently. Mostly cause they are not following the W3C. The biggest thing to remember is that your site should be built so that it is viewable the way you want by the most popular browsers.
Currently the most popular browser is Internet Explorer though traffic from Apple’s Safari and also Firefox are gaining ground. So if your gonna write website code then you need to have a few browsers ready to look at the pages you create so you can see what it looks like before you post it to the internet.