The National Occupational Classification (NOC) is Canada’s national system for describing occupations. You can search the NOC database to find where an occupation is classified or to learn about its main duties, educational requirements, or other useful information.

The NOC system was introduced because IRCC, ESDC, and other arms of the govt need a common way denominator to assess job functions. Moreover, there is a need to assess work experience in most immigration programs. Unlike education credentials and language assessments, this assessment is still done by the visa officer. This database is revised once in a while and has been revised thrice, in 2006, 2011 and 2016. The latest version among them is the 2016 version. This is currently in use.

How do I interpret the NOC classification?
NOC codes are 4 digit codes that correspond to your specific occupation. The first two digits show your broad category and skill level. These two together are known as the major group.
Here is an example, for instance, if you are a software programmer, you can see the below breakdown of your NOC code - 2174.
The major group is a combination of the broad occupational category - 2, and the skill level - 1. The minor group is a combination of the major group and the subcode specific to your occupation subcategory - 7. The 4th digit in the NOC code identifies your exact occupation within the occupation subgroup - 4.