HTML is used to create web documents and consists of text and markup tags to define structure, appearance, and hyperlinks. There are two types of tags: container tags define sections of text using start and end tags, and empty tags represent single occurrences like line breaks. CSS is used to style HTML documents and consists of rules with selectors and declarations specifying properties and values to control styling. PHP is a widely used server-side scripting language with roots in C and C++ that is commonly used with MySQL, a popular open-source database, to create dynamic web applications.