HTML and JavaScript are client-side scripting languages that are executed by the browser as the page is loaded. CSS is similar to this and is linked to HTML documents, it is primarily used to style the HTML.Used properly, HTML presents and structures the content that is to be viewed, JavaScript can add extra functionality (and do much else) without causing usability or accessibility issues, in fact if used intelligently and creatively it can increase usability. CSS is there to make the page look good, it has other uses but exists to style documents.

XML (eXtensible Mark-up Language) is a general purpose language and it allows users to define their own tags/elements. The following are application languages of XML: XHTML, RSS, MathML, GraphML, SVG. A knowledge of XML would be beneficial but is not really necessary.

PHP is a server-side language that is executed before the document is sent to the client (browser). It allows you to create dynamic content on the fly by either pulling information from databases and placing it into a html document or by creating content based on information that the user has provided or often combinations of these. PHP is a programming language, the others previously mentioned are not. It is necessary to embed PHP into your HTML for it to function properly within the context of a website.

These are (or can be) your main tools as a web developer. They can be very powerful in the right hands and can produce some impressive applications but it is often unnecessary to use all of these languages. Use JavaScript sparingly and with accessibility in mind, and use it to enhance the user experience. Always ask yourself "IS this necessary?" if you can honestly answer yes then you should be ok.

Pete.