OS X doesn't come installed with software to open Microsoft Word documents. And unlike on Windows, Microsoft don't provide a Word Viewer for Mac.
Fortunately however, there are other ways of opening Word documents on Mac without having to purchase Microsoft Office for Mac. Here are a few of them:
OpenOffice.org is a multi-platform and multilingual office suite that is compatible with all other major office suites (meaning it will open and edit Word, Powerpoint documents etc). Best of all, its absolutely free to use.
Like OpenOffice, LibreOffice boasts a comprehensive set of office management tools arguably on par with Microsoft Office itself and it opens Word documents. LibreOffice offers six different office management tools to meet your project management needs: Text Document, Spreadsheet, Presentation, Drawing, Database, and Formula.
Pages is Apple's answer to Microsoft Word although it can take some getting used to if you've used Word all your life, once you get the grip of it it is easy to use. However, it's not free - it comes as part of iWork which includes among other things, the excellent Keynote presentation software - but it is relatively inexpensive compared to Microsoft Office.