why cant we create folder name with some words??

In MS-DOS, several special "device files" were available to aid in performing certain tasks, such as clearing the screen or deleting extraneous output from a program. In order to maintain backwards-compatibility, all versions of Windows up to and including Windows Vista will refuse to allow you to create a file with these "reserved" device file names. The following file names are reserved:

• CON
• PRN
• AUX
• NUL
• COM0
• COM1
• COM2
• COM3
• COM4
• COM5
• COM6
• COM7
• COM8
• COM9
• LPT0
• LPT1
• LPT2
• LPT3
• LPT4
• LPT5
• LPT6
• LPT7
• LPT8
• LPT9

CON, incidentally, was a device file used to capture whatever was printed onscreen.
You have to study the subject of "reserved words" in windows. All the following are reserved words: The CON is a reserved word for the console CON, PRN, AUX, CLOCK$, NUL COM0, COM1, COM2, COM3, COM4, COM5, COM6, COM7, COM8, COM9 LPT0, LPT1, LPT2, LPT3, LPT4, LPT5, LPT6, LPT7, LPT8, and LPT9.