HTML vs. PHP vs. ASP
You may notice when you are browsing websites, that some of the pages end with html, php, or asp, but what exactly does this mean? To the person browsing the site, it doesn't mean anything really. The difference between these types of files is how the web server handles giving your browser the information.
HTML is the simplest form. These are files store on the server and when you request a page ending with html extension, it gives your browser an exact copy of what is saved on the server. You could view the source of an HTML document and then save it to your machine and it would always look the same.
PHP and ASP work differently. These are files which have code the server understands, so when you request a php or asp page, the code is run and your browser is sent HTML code which you see. This is how interactive web pages work. The difference between the two is just the code language used. ASP is Microsoft's format which is based off of Visual Basic and only works on Microsoft servers. PHP is an open source language which looks similar to Java and C and works on any server platform.