EPUB can, in principle, represent anything that you might deliver via an HTML5 website as long as the core content assets (HTML and CSS files, images and other media) can be determined in advance so that they can be stored in the .epub file.
In practice, the structure imposed by EPUB which includes a requirement for a linear reading order and a table of contents, means that the EPUB format does not provide much value-add as a delivery vehicle for arbitrary web applications. Movies and interactive games, for example, could potentially be delivered via EPUB but there are other means to package such experiences.
EPUB is well-suited for all kinds of documents, aka publications. These have a reading order and the concepts of a table of contents and navigation are applicable. EPUB has rapidly become the prevalent format for eBooks, digital editions of novels and other types of books generally found in bookstores. EPUB is now being adopted for e-textbooks, digital comics and manga, digital magazines, academic journals and other scientific and scholarly publishing and corporate documents and brochures.
This adoption across many segments of publishing is significantly enabled by the newest version of EPUB, EPUB 3.