The internal recruitment sources are very important, but they cannot be used to fill every vacancy in the organization. It is very important to realize, that in many organization, the internal recruitment is divided into two separate processes: Internal recruitment and promotions. The promotion is the move of the employee when the organization initiates the whole process. The real internal recruitment is than a move of the employee initiated by the employee him or herself.
The external recruitment sources bring job candidates from the external environment using different techniques. The oldest, but still pretty efficient is a newspaper job advertisement. Many HRM Professionals do not believe in the power of the newspaper advertising, but for many jobs it is still one of the best techniques with the best cost/income ratio.
The modern recruitment source fully managed by the organization is the web job advertisement. It is very cheap, but it can flood the organization with many useless job resumes. This can make the final decision almost impossible. The cost/income ratio is always attractive, but the success rate can be really low.
The other external recruitment sources are the recruitment agencies and executive search companies. Their services are not cheap, but the organization does not have to handle all that job resumes and the good agency also makes follow ups of the job candidates in the recruitment process.
A very special kind of the recruitment source is the referral recruitment, when the employees are paid to provide the organization with their friends as potential employees. This can look strange, but it can provide the organization with many interesting candidates, who are not reachable via any other recruitment source.