An Online Invitation or Online Save the Date come with an email that you send to all your guests, plus a digital card + event page. The email you send will link to your digital card + event page. On your event page you can choose to collect RSVP's or link somewhere else (your wedding website, for example). The event page also offers the ability to add a map, show your guest list, have a comment wall - things like that.
The Wedding Website Announcement is only an email, and you cannot collect RSVP's with an Announcement. You will design a beautiful email that you send to your guests, and that email can link somewhere (wedding website, for example) or nowhere at all. Announcements are often used for event updates, thank you cards, RSVP reminders, save the dates, etc.