So, one day i type in .com in extension for file, while not paying attention really so much. Then it turns into a ms-dos
application.

yes, .com files are/were executable files within the DOS environment. So what is your question?

are/were? Im on windows 8.1. Just wondering the relation between .com and ms-dos. Why not .msdos?

Because back in the day when computers had 4K of RAM and a 40MB drive was huge, you needed a small compact program. And a .COM was it. It's a throwback from yesterdecade.

@Leo G - this goes back further into the days when MS-DOS was first derived from Seattle Computer Product's QDOS opearting system that MS purchased to sell to IBM for the first IBM PC. In those days, storate was on a small (underl 1MB) floppy disc. So, your comment about .com files being a small compact program is spot on. They still will run on Windows systems, but most are now .exe files that have more capabilities. Like a zombie, you just can't kill them! :-)

FWIW, I think (not 100% sure) that .com programs are still 16bit software, whereas .exe programs can be 32bit. Before NT, the Windows command processor was cmd.com. Now it is cmd.exe, with much greater capabilities, at least as far as utilization of system resources is concerned.