SCREEM is a web development environment. It's purpose is to increase productivity when constructing a site, by providing quick access to commonly used features. While it is written for use with the GNOME desktop environment in mind it does not specifically require you to be running it, just have the libraries installed.
Unlike most other web site / HTML editors SCREEM does not provide a WYSIWYG display of pages. Instead you are presented with the raw html source in its editor window.
In general WYSIWYG editors do not produce good clean valid HTML, and can also slow you down if they do not support an element that you wish to insert. By utilising a text based editing system you can use the markup you want rather than what the application thinks you need, and also provide quick access to commonly used elements via toolbar buttons which insert the markup at the current cursor position.
In addition to excelling as an HTML editor, SCREEM also lends itself to being a versatile XML editing package, perfect for use as a general purpose structured data editing environment.