Microsoft Office is a suite of productivity applications that let you perform common tasks like produce documents, spreadsheets and presentations. While Office is usually bundled with new machines buying it as a stand-alone product can be very expensive. That's why you should consider other alternatives before you make your decision.
One free alternative is called OpenOffice. OpenOffice is available as a free download from www.openoffice.org. It runs on all platforms (Mac, PC and Linux) and can talk the same language as other office suites, including Microsoft Office. That means you can exchange files, including those with .doc, .ppt or .xls extensions with others who are using Microsoft Office. As well as the regular office features, OpenOffice features also features a database program called Base and a program called Math that easily lets you create mathematical equations.
Another free alternative to Microsoft Office is called ThinkFree (www.thinkfree.com). Like OpenOffice, ThinkFree features a word processor, spreadsheet and presentation applications. ThinkFree is also compatible with Microsoft Office. Another nice touch with ThinkFree is the ability to save any ThinkFree document in to PDF format. You can do this with all the other packages as well but it requires installation of extra software. ThinkFree also offers ThinkFree Online which is an online version of its ThinkFree package and includes all the main packages.
Lastly, if you don't need a presentation package (like PowerPoint) then I suggest you try Google Documents and Spreadsheets. You don't need to download any additional software, just go to docs.google.com, sign in with a free account and start your document or spreadsheet. While Google Docs isn't a full featured word processor or spreadsheet program like ThinkFree Online I think it does most of the basic functions well enough, and you get the benefit of having all your documents backed up online all the time. Also, if you've already got a Google account you can access your docs by clicking the docs link at the top of any Google page.