OS X doesn't come installed with software to open Microsoft Word documents, and unlike Windows, Microsoft doesn't provide a Word Viewer for Mac.
Fortunately, 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 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. Mostly importantly, however, 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. Unfortunately, it's not free– it comes as part of iWork, which includes, among other things, the excellent Keynote presentation software. That being said, it's relatively inexpensive compared to Microsoft Office.
Looking for more options? Google now offers a full suite of apps, including Google Docs, Sheets, Slides, and Drawings, which could be the next great replacement for Microsoft Word, compatible with all OSes and based in the cloud.