What is a gang?

A gang is usually considered to be a group of people who spend time in public places that

see themselves (and are seen by others) as a noticeable group, and
engage in a range of criminal activity and violence.

They may also have any or all of the following features:

Identify with or lay a claim over territory
Are in conflict with other, similar gangs

However, if the majority of offending is of a lower non-violent level then they would be considered a peer group not a gang.

A criminal network (which is different to a gang) is:

A group of individuals involved in persistent criminality for some form of personal gain this includes profit and/or to gain or demonstrate status) which is causing significant harm to the community

A group that keeps breaking the law to make money
This law-breaking is causing harm to the community
or, this law-breaking is a problem internationally (e.g. people trafficking)
violence is used in order to make money (e.g. to scare people into giving them money)
They are running an illegal business (e.g. drug trafficking)