LibreOffice is an office suite that runs on Linux, macOS, and Windows. It is released under a free license that lets you use it legally on as many machines as you want, and share it with others. Since it is free software, you can even make your own changes in it if you have the skill.
LibreOffice is descended from OpenOffice.org, a free and open source software project run for years by Sun Microsystems. When Oracle bought Sun in 2010, LibreOffice started developing its own version of OpenOffice.org, as the code’s license permits. Eventually, Oracle gave its rights in the code to The Apache Foundation.
Today, OpenOffice.org no longer exists, but Apache OpenOffice does. LibreOffice and Apache OpenOffice are similar in many features, but after a dozen years of independent development, may no longer be completely compatible. LibreOffice has many features that OpenOffice lacks.
Several other organizations repackage both LibreOffice and Apache OpenOffice, sometimes under different names. This practice is perfectly legal, but sometimes involves using different icons and making other minor changes.
What is Open Document Format?
Both LibreOffice and Apache OpenOffice use Open Document Format (ODF), a file format consisting of zipped XML files.
You can use this format to exchange documents between the two, or any other application that uses ODF, such as Calligra Suite or Abiword. In both LibreOffice and OpenOffice, you can also open and save to most Microsoft Office formats (but OpenOffice cannot save to current Microsoft Office formats such as DOCX), and open files from WordPerfect, Microsoft Works, and several other formats, although some formatting is sometimes lost in complex documents.