A radiobutton looks similar to a checkbutton because it also has an indicator on the left side. The radiobutton indicator is a diamond, rather than a square. Both look 3D and are slightly raised when unselected.
The main difference between a radiobutton and a checkbutton is the function they serve in an application. A radiobutton is used to select one of several different choices:
In a multiple choice test, the answers A, B, C, D, or E
Which version of a tool you would like to use
Your income range: 0-20,000; 20,001-30,000; 30,001-40,000; 40,000 and up
Which type of entree you prefer: beef, chicken, or vegetarian
In each example, only one answer is appropriate. For instance, it wouldn't make sense to have a salary of both $18,000 and $33,000. And when you are taking a multiple-choice test, you can't select all the answers and hope that the teacher gives you credit. You have to pick only one.
Because radiobuttons are used to decide between several choices, you should always create at least two. It doesn't make sense to ask a question at all if there is only one choice. Radiobuttons should always be created in groups of two or more.
 If you did create only one radiobutton, it would start out unselected (unless the variable you associated with it contained the on value). Once that radiobutton was selected, you would never be able to deselect it.