LyX is a "What You See Is What You Mean" text processor. Or word processor. Or a cross between the two. Or a front-end for LaTeX. Or all of the above.
What it can be is a replacement for your average word-processor written by a bloated multinational corporation which shall remain nameless. It runs on both Unix/Linux and on Windows (if you have Cygwin). Since LaTeX is the de-facto standard for publishing in Academia, and has a very long history of excellence, you don't need to worry about how good your typsetting will look. But with LyX, you don't have to be come a Master of LaTeX to produce your documents.
Enough ranting. You can find out more on the official LyX page.