Both are online calendars that can be viewed by your clients through a web browser. The Web Roster shows scheduled classes such as "Monday 9:00 AM, Zumba, Metropolitan Gym with Roger", whereas the Availability Calendar just shows when you are available and when not (white vs gray cells). In the Web Roster clients can request to be added to an existing (group) class and in the Availability Calendar clients can request an appointment in a certain time slot. If you have defined different activities and instructors, then the client also needs to pick from those.
Which calendar is best for you and your clients depends on your type of business: Group classes or private appointments. You can use both of course.