Registration system in different web sites

I found a number of registration systems in different web sites. All are developed differently. Different as a sense of different fields and techniques. But all web sites main concern is genuine and valuable members/customers. Registration fields are always depends on the type of web site. Registration system is almost endless. If you like to add more and more features you can. If think make it simple, then simple and meaningful. Some times memberships are controlled by admin, without admin permission registration is incomplete. Here i mention some registration system.

Simple registration : Have only few fields like First Name, Last name, Email, Contact Number, User name and Password. After submit you are registration form you are registered member and can enjoy all facility of registered members.
Slightly improved version : uses email verification. They send a secret code to your email. By clicking the verification link you have to confirm you are the owner of this email account. It's a good validation maintained in registration system. Otherwise someone might use your email id and later you can't register by your own.
Membership approved by Admin only. This type of registration contain as usual fields with email verification. First you register and verify your email address. Then admin view your profile (usually through admin panel) and contact with you (via phone, email or any other way). Finally admin approve you as a member. Admin have full right for disapprove/block your membership at any time.
Secured registration : Image verification used at number of sites for registration process. This type of verification use for protect automatic registration (by software).
Paid Registration : Most of the web sites registration is free of cost. but few sites take change for membership. Few site says pay once and get membership forever. And one other category take charge for a period of time.

All these registration are seems same for site visitors. Because of the design and representation of user interface(UI). Functionality are different for different systems, what is almost hidden from users.

Registration is an important part for any web sites. Whatever it's popular like BBC, CNN or new www.novice-developer.com