Help with DOS syntax for file deletion

I am looking for the correct DOS syntax to achieve the following:

In folder "C:\ABC":

1) Delete ALLA folders, including their content.
2) Delete any files that are in the root of "C:\ABC"

I have tried a combination of RMDIR and DEL but I am not managing to delete folders.
I can't find any examples of syntax online.

Any idea how to achieve this?

Technically its not DOS, its a command shell. Have a look at this, Del Try del /? for a list of options.

Use DELTREE
Syntax : DELTREE [/Y] [drive:]path [[drive:]path[...]]

So in your case : DELTREE /Y C:\ACB
It will delete the ACB directories and all its subdirectories, files included. The /Y switch suppresses the prompt that asks for confirmation of each folder's deletion.
You'll have to recreate the directory though. You can still do that under the command shell, type :
C: (redirects you to drive C)
CD \ (redirects you to the root of the drive)
MD ACB (creates the directory)

Be careful with DELTREE, it can be very dangerous if not used right.

Edit :
My bad, DELTREE doesn't exist anymore in this version of windows (still existed under XP I think, sigh). Here is the equivalent now :
rd C:\ACB /s

Use: rd /S /Q [dir] ======> [dir] is the folder you want to remove => remove everything in [dir] including [dir] itself.