By default, there are five member groups. You are already familiar with the Super Admins member group (which you are a member of).
The Members member group is the group that approved members are put into (unless you have changed the default member group in the member preferences). By default, members in this group can do everything available on the member screens (such as sending private messages) as long as you have not disabled the option in the global Member Preferences. However, members cannot log into the control panel.
The Pending members group is for members who have registered but have either not activated their membership via e-mail (if you allow self-activation by e-mail) or have not yet been manually approved ...
No credit card required