LyX is a structured documentation system that uses LaTeX, a typesetting system, as its back-end. This means that you enter the content and the system works out the best layout. As the user does not have direct control over layout, LyX isn’t optimised for free-form documents such as posters, but it comes into its own in the creation of longer, more complex projects such as a book.
The LyX interface doesn’t qualify as WYSIWYG, but it does reflect elements such as emphasised text, and it does allow you to see images (called figures in LaTeX-speak) and tables within the document. Although it is a front-end to a markup system, you can typically complete a complicated project without ever encountering any LaTeX code, but you can dabble with it to add customised effects.