What is the difference between a crew and a gang?

Gangs committee Illegal acts, as where crews are a group of people necessarily not committing illegal acts.

Some Crews committee illegal acts saying they are Crew to not come off as a gang. But if a crew with hand signs commits these acts they law considers them as a gang.

Examples

"Lets kill that Gang" - Illegal Act

"Get the Crew together for the party" - Just going out as a group