The .docx extension is for the Ms-Word 2007 File and .doc extension is for the Ms-word 97-2003 or earlier Files. Many Users who are using Ms-word 2007 for the first time, they get confuse about the difference between the .doc and .docx files. Actually when i was using ms-office 2007 for the first time i got the same doubt that is what is the difference between .doc and .docx files. Both are ms-word files only.
Microsoft has given .docx extension for the ms-word 2007 or later version files. However Ms-Word 2007 supports both .doc and .docx files. Ms-word give us a option to save as .doc or .docx as our wish. To see this click on office button->save as , there you will find the drop down list “save as type:” which gives some options to save the file as we want whether it may be .doc or .docx or .txt etc..
We can open a .doc file in word 2007 but we cannot open .docx in Ms-Word 2003. However Microsoft is providing some converters to open, edit new files in a old Ms-word. For more details about those converters click here