COM files were the original MS-DOS executable binaries (before EXEs became the norm) and are rarely used anymore. However, Windows will still give preference to COM files over EXEs when a file is referenced without extension, which can be valuable to clever exploit developers and malware writers.
To see how this works:
1) Open a command prompt and type "notepad" to launch the notepad.exe application from system32.
2) Open explorer and browse to C:\Windows\System32
3) Copy C:\Windows\System32\calc.exe to C:\Windows\System32\notepad.com
4) Close out of the notepad application and run the "notepad" command again in the command prompt.
Once the 'notepad' command is run Windows will search System32 for the appropriate application, find the "notepad.com" file and notepad will get hijacked by calculator.
This technique is used less often since most programs are launched with the file extension now but if you see COM files in strange locations it might still be worth looking into.