.doc is the generic file extension for documents created with Microsoft Word. Even though the format is proprietary (and owned by Microsoft), it has become de facto standard for (formatted) text documents even though open alternatives would exist.
It should also be noted that the file extension .doc itself doesn't give out any information about the version of the Word that the file was saved with -- and typically older versions of Microsoft Word can't open files generated with the latest versions of the same software (unless the creator has specifically stated he/she would like to save the document using an old Word format).
Despite being a proprietary document format of Microsoft, .doc can be opened with most of the alternative office tools, including the open source OpenOffice.org.
Related software tools
Apache OpenOffice (Open source)
OpenOffice.org is is a multi-platform office productivity suite.