The process of focusing one's occupational concentration on a specific area of expertise. An increase in job specialization among employees can make them less flexible since it tends to reduce their ability to perform other types of work within the business that fall outside their particular specialty.
Head hunters tend to favor the resumes of those who demonstrate a wide variety of interests and skills over the resumes of candidates that display job specialization and experience in only 1 part of their field.
Source: http://www.businessdictionary.com/definition/job-specialization.html