MS Word 2007 uses a .docx extension by default.
.png is the default extension of an Ms-paint file
For MS Word 2003 & earlier versions: .doc For MS Word 2007 & next versions: .docx.
Version 2003 - .doc , version 2007 - .docx
For a standard Excel document, prior to Excel 2007, it is xls and xlsx is used in 2007. There are other kinds of files created by Excel, like templates which can be xlt or macros which can be xlm and xla for Add-ins, and there are other extenions. For 2007, you add x to the end of them.
.docx
.docx is the default file format for documents from Word 2007 onward.
The standard extension for most versions of Word has been .doc but in Word 2007 .docx is used.
The file extension for Microsoft Access 2007 is '.accdb'.
.accdb is the file extension for Microsoft Access and has been used since Access 2007. The previous file extension was .mdb.
There are no differences between MS Word 2007 and MS Word 2007. I suspect you will have another question after you read this answer.
.bmp, by default, as in bitmap.
.pptx