A ballerina is a name given to a female ballet dancer. Only very good female ballet dancers are called ballerinas. A ballerina is a principal dancer in a classical ballet company. The highest ranking female dancer in a company was usually called the prima ballerina.
Today the term 'ballerina' has gone rather out of fashion. Now they are simply called 'Principal dancers'. Dancers in a company are ranked by the company, and paid accordingly. They have to prove themselves in lesser parts before they can get promoted.
Example of the ranks in a modern dance company: