That depends on the type of file. The main one is .mdb which stands for Microsoft Database.
.accdb is the extension used for Access 2007 and 2010.
.mdb is the file extension used by Microsoft Office's Access program and it marks a file that is a database.
.accdb is the file extension for Microsoft Access and has been used since Access 2007. The previous file extension was .mdb.
A .ppt file is a file to be used in the Microsoft PowerPoint software.
.docx The extension docx is used in Word 2007.
.dbs
".doc" in versions of Microsoft Word before 2007".docx" in Microsoft Word 2007 and later..docMS word
the Microsoft word files are called document
The olp file type (extension) was used by Microsoft for Office Live Package(?) to allow for the backup and restoring of Office Live websites.
xlsx is the main extension used for Excel 2007.
The CUR file extension is only used by Microsoft's Windows operating system for cursor images.
This is going to depend on the type of file that you are trying to open. The file extension is created depending on the software (and version) that was used. For example, if you use an older version of Microsoft Word to save a document, it has a .doc extension. If you use a newer version such as 2007, it will save the files with a .docx extension. You would need to have the correct software to open the file, but in many cases, Microsoft may make a viewer that you can download (depending on the file of course)