LaTeX (pronounced latek and not latex, just so you dont sound stupid)
Best way to describe it is a form of markup language, commonly used (well within computer science anyway, dont know about other fields) to write documents, reports etc...
If you look at a webpage the layout is described by the html. It is a similar idea for MS Word documents, when you write a word document it automatically adds the formatting info to the file. you cant see it because you dont want to see it, you want to see the formatted text. - try opening a word document in a text editor.
In LaTeX you write the formatting code along with your document and then compile it to get the resulting document. A bit of a hastle if you have not used it befor because you have to learn it. When you get to know it you can do some really advanced things. Unless you need to write Z-notation or something particularly mathematical and are prepared to spend time learning it you are perhaps better sticking to your current word processor