LibreOffice isn't quite the same as OpenOffice, although it was originally derived from it. Most development seems to be going into LibreOffice now.
Both use Open Document Format as their native file type but can also save in MS Office file types too. An advantage of .odf is that the file is just a .zip file so if something goes awry you can often access your data using standard .zip compression tools. Formatting is kept separate from the data so if you burrow through the .zip's file structure you can access the text. You might lose the formatting, but at least you have the possibility to copy & paste the data.
Some things I prefer about MS Office, others LibreOffice. I can't stand the MS ribbon so MS Office 2003 is the latest I use.