Microsoft's Office suite of products is the industry standard. If you will be sending any documents to other businesses, you are almost certainly going to need MS Office. Office comes in a varitey of options, from the basic, which just includes Word, Excel and Outlook, through to including all sorts of other things.
Alternatives
There are alternatives and many are great options if you
don't have to send standard documents to other businesses or clients. Open Office
is a freebie that is almost as good and has high compatibility with MS
Office. It will also run on Windows, Mac or Linux and is available in a
number of languages.
Mac's have their own version of products and are generally interoperable with MS Office.
Mozilla has a product called Thunderbird, which is comparable to MS Outlook.
Google has GoogleApps, which is an online version of MS Office like products. Great if you have a mobile workforce, a little limited if you don't.
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