So what is Google Docs? It's free online storage for your office files (documents, spreadsheets, presentations). Plus online editing of office documents, with no desktop application required. So you don't have to carry your files with you, or sync them between different computers -- you can store and organize and edit and share them, all from the online cloud (though there are some limitations depending on the file formats).
You can share your documents with others, by invitation or through authorized Google Accounts. You even can collaboratively edit documents with multiple people.
And there's more -- Google Docs is also a great way to share your existing files (and collections of documents) without needing to convert them to HTML or manage multiple versions in different formats.
You can Publish documents to a webpage, and Google Docs converts your file to formatted HTML, and returns a custom URL to the page. You then can share the link privately by e-mail, or post it on the web for anyone to access.
To build your online collection of files, you can upload (and download) files in a variety of formats, including documents (Word, OpenOffice, RTF, HTML text), spreadsheets (xls, .csv, .txt and .ods), and presentations (.ppt and .pps). And you can export to PDF and HTML. Or upload documents by e-mail by sending to a custom e-mail address -- as the body of the message or with attachments.
Google Docs is part of the Google Apps suite of Web services, including Gmail, Google Talk, and Google Calendar, and works with the single Google Account login.
Of course, for Web communicators, Google Docs is also great for linking documents from your posts on Google's Blogger service -- for free, simple, but customizable blogs.