A crowd is a group of people. The crowd may have a common purpose or set of emotions, such as at a political rally, at a sports game, or during looting, or simply be made up of many people going about their business in a busy area (eg shopping).
In human sociology, the term "mobbed" simply means "extremely crowded", as in a busy mall or shop. In animal behavior mobbing is a technique where many individuals of one species "gang up" on a larger individual of another species to drive them away. Mobbing behavior is often seen in birds.
It is generally considered that a crowd consists of more than 20 people of which no more than 10 can be your friends.
Social aspects are concerned with the formation, management and control of crowds, both from the point of view of individuals and groups seeking to persuade a crowd to their view (e.g., political rallies), and from the point of view of society which usually attempts to contain them in an acceptable manner, or discharge their energies whilst averting excesses or mob behaviour, ultimately a decision made politically and usually executed by law enforcement.
Psychological aspects are concerned with the psychology of the crowd as a group and the psychology of those who allow their will and emotions to be informed by the crowd (both discussed more comprehensively under crowd psychology), and other individual responses to crowds such as crowd-sickness, claustrophobia and agoraphobia.