HAML is a markup language that simplifies HTML by removing angle brackets and unnecessary tags. It aims to make markup easier on the eyes and fingers by reducing the number of characters needed. Key features of HAML include self-closing tags through indentation, optional closing tags, ability to nest layouts and partials properly, and automatic generation of valid XHTML. HAML code can be syntax highlighted and edited in tools like text editors, IDEs, and plugins for Rails. It is available as both a Rails plugin and Ruby gem.