When you call a group of people a mob, you are saying that they behave in a non¬-thinking, or perhaps violent way. The word mob can also be used as a verb. You can mob someone or something by surrounding it in an angry or threatening way.
Ex: The crowd was beginning to take on the characteristics of a mob, with growing volume and increasing anger and threats.
Ex: The crowd mobbed city hall just in time to see the mayor whisked away by his driver in a limousine, making it even more angry.
In English, the mob is a slang term for the mafia, or organized crime. It may also be used as an adjective describing things having to do with the mafia or organized crime. When a group of people stage a performance in a public setting that is meant to look as if it was improvised, it is called a flash mob.
Ex: In the 1980s John Gotti and other members of the Gambino crime family were jailed, putting a dent in the mob's ability to control New York City commerce.
A swarm is group of moving people or things, sometimes surrounding an object or person. The word swarm can also be used as a verb, meaning to move around or toward something in a group.
Ex: The swarm of teenagers crowded at the stage door to beg for autographs as the pop star emerged from his concert.
Ex: As soon as the sale opened, the crowd began to swarm the displays, looking for inexpensive but warm clothing for the winter months.
The term swarm is often specifically associated with bees, termites, and other insects as well as their movement.
Ex: The young boys, one of whom was allergic to bees, ran into a swarm that had just been disturbed by the boys in front of them.
Ex: As he watched the honeybees swarm the hive, the inexperienced visitor began to be nervous about helping the beekeeper with his task.