When you are reaching out, you could be reaching out to a person or a website. We collectively call this 'prospects'.
A prospect is a person or a website that you are reaching out to with an offer or an "ask" in order to get some value. For example, you may generate a lead from the interaction.
In an ideal world, you would have the contact of a person when you're reaching out, but sometimes you don't. So instead you may find the contact form on a website and send an email through this form.
You may also have contacts that do not have a website! That's why prospects are split up into websites and contacts.
If you create a website with contacts, you'll see it listed as a website with contacts under the website tab. You'll also see them as contacts under the contact tab.
If you create a contact without a website, then they will only be visible under the contact tab.