I assume you mean worker recruitment for business hiring. Internally, often times a workplace will transfer personnel from one department to another because they recognize talents or potential that could better serve the company in another area, or they want to train and develop key personnel with new skillsets. Especially in management, most companies hire from within because the workers they already employ are known quantities.
Externally, often times companies will rely on Temp agencies that screen employees for them and are used to fill specific tasks or short term jobs that don't receive the better pay and benefits of a long term hire.
Other external sources include the internet in terms of advertising and job search sites, local chambers of commerce, employment agencies and newspaper classifieds.