This one’s easy, but I’ve just been asked by someone new to Linux so here it is:
$ rm -r <path to directory>
So for example if you want to delete a folder called temporary from your home directory:
$ rm -r ~/temporary
The -r switch stands for recursive, so you’re not actually deleting the directory with the files within it, you’re deleting each file within the directory first, then removing the directory afterwards.
Simples.