Any website can be submitted to regular directories. These directories have all the main categories, sub categories and inner categories. The website is submitted by choosing the right category.

Niche directories are exclusive directories which allow websites related to a particular category. For e.g. a niche directory for Legal will not have a category for "Sports" or "Health". They will only have sub categories which fall under Legal like "Law firms", "Divorce Attorneys" etc. So it is possible only to submit related websites there.