OpenOffice.org is a multiplatform and multilingualoffice suite and an open-source project. Compatible with all other major office suites, the product is free to download, use, and distribute.
Put it simply, Open Office does the same thing as Microsoft office with one exception, its FREE.
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Historical background
StarDivision, the original author of the StarOffice suite of software, was founded in Germany in the mid-1980s. It was acquired by Sun Microsystems during the summer of 1999 and StarOffice 5.2 was released in June of 2000. Future versions of StarOffice software, beginning with 6.0, have been built using the OpenOffice.org source, APIs, file formats, and reference implementation. Sun continues to sponsor development on OpenOffice.org and is the primary contributor of code to OpenOffice.org. CollabNet hosts the website infrastructure for development of the product and helps manage the project.