HTML (HyperText Markup Language) and the extended version XHTML are languages which are used on webpages (they descripe what is on the page, i.e. e.g. where the text (which) should be) and are understood by web browsers. So either HTML and XHTML are used on all pages displayed by a web browser. Sometimes these pages are automatically created by scripts.
Dreamweaver is on the other hand a software to create homepages. This program is used much by professionals. But in you only need an editor to create web pages (l use e.g. Bluefish and their are other freely available ones).
PHP is a script language (see e.g.
http://www.php.net) which is used on a web server to create dynamical homepages, e.g. pages which are using information from databases.
Many pages are using so called java applets (but also JavaScript). The reason I guess why Java is used is because it is more portable. But PHP has many similarities with C.
I hope this information is helpful.