General Characteristics
From a psychological perspective, a group of individuals is not sufficient to have a crowd, but it must evidence specific characteristics. Specifically, the people in a crowd cease to think and act as individuals, but become part of an amalgamated group. They abandon their identities and individual motives and become obedient members of a herd whose primary motivation in the moment is in supporting or imitating that of other members of the herd, without considering themselves as individuals.
The size of a crowd varies: it may be as few as half a dozen men (a gang) or as many as (seemingly) an entire nation The crowd may exist as a crowd for a short or long period of time, and may even disperse and reform itself. It may consist of smaller crowds that unite and disunite in their specific purpose.
A crowd is most often assembled by chance, though there are instances where a crowd may be assembled - people purposefully go to join a crowd, or people call to others to join a crowd. A crowd may be assembled for "heroic or criminal" purposes, but does not always follow the purpose for which it was assembled. The crowd makes decisions and directs its own behavior, and the intentions of any organizer are incidental.
(EN: This problem exists in crowd psychology here and elsewhere. I've seen the distinction made between a "mob" and an "organization" to distinguish between crowds that have a clear purpose and structure and those that do not.)
You may, in any city, walk the streets with dozens, hundreds, or thousands of people, each proceeding to his own destination and largely unconscious of others except as moving objects in his environment, following various conventions not to jostle one another or get in each other's way. This is not a crowd, but merely a mass of people without a common purpose.
Le Bon briefly mentions that there are different kinds of crowds, and promising to consider their classification alter in the book, suggest that instead he will deal with them in general in the early chapters and explore the distinctions among their species in later chapters.
Crowds, like people, are highly idiosyncratic, and even a defined crowd may change its characteristics. Moreover, the character of a crowd may be entirely different to the character of the people who comprise it. It is often observed that member of a riot are inoffensive people who under normal circumstances are entirely docile and law-abiding citizens.
This inconsistency between the group and the individuals who compose it is a common characteristic of crowds. The notion that the crowd is an "average struck between its elements" is entirely misguided - just as in chemistry the combination of elements results in a compound that has entirely different properties to its constituent elements.
Psychologically, the crowd is not the product of the conscious mind. Those in the crowd do not act because they are exercising reason and logic - they are merely reacting to one another, in a very instinctive and animalistic manner. It can be observed that a crowd of human beings behaves little differently to one of lesser primates.
It can even be observed that, in a crowd, individual distinctions become muted. A learned professor and a common laborer follow much the same behavior patterns when they are swept up together in a mob. It is not so much a collective mind, as all minds being reduced to the lowest common denominator of human conduct.
This very fact is the reason that no crowd has ever been witnessed to accomplish any action that demands a high degree of intelligence. A crowd is an accumulation of stupidity and emotion. (EN: The same is often observed of groups such as committees and work teams. The term "groupthink" was coined to describe the phenomenon by which a group of intelligent men can make a less intelligent decision than any one of them might have made on their own.)
Rashness and overconfidence is a significant factor. A mature and experienced individual tends to have some doubt in his own reasoning that leads him to be reluctant. But in a crowd, the large number of people who seem to be in agreement reinforce one another's half-witted ideas, and from their sheer numbers have a sentiment of invincible power which gives them boldness to take action. A crowd never questions whether an idea is good - that it is popular is sufficient to motivate them to act upon it.
Contagion is another characteristic of crowds, and one that is not so easy to explain. Some voices in the crowd are ignored, but others spread wildly, and the rest follow in a hypnotic manner. For an individual, an idea is adopted or a suggestion is followed because it makes sense - whereas in the mob reason plays little role, and it is not entirely clear what does cause a statement to be repeated, to the point of being chanted by all.
(EN: Backs to modern psychology, it's suggested that people are more confident in following the ideas of others than suggesting their own. There is less risk to the ego should it be contradicted - the blame falls with the person who said it first, not the one who repeated it. And in a crowd, it may be impossible to know where something originated.)
He goes further to consider the similarity between crowd psychology and hypnosis: people speak and act without thinking, following the lead of others without considering the goal or even the sagacity of their actions. Once a person gives himself over to the will of the crowd, he is no longer acting in a state of full consciousness. It is a kind of conformity, in which one person does as others are doing, simply because others are doing it.
In a very literal way, a man in a crowd "is no longer himself, but has become an automation who has ceased to be guided by his own will." He may be a civilized man, but becomes as a barbarian, acting by instinct with spontaneity, fearlessness, and ferocity.
We see the same even in organized crowds, which are supposed to have purpose, and which are ostensibly gathered so that the intelligence of many men may be applied to a problem. Juries deliver verdicts of which each individual juror will later disapprove, parliamentary assemblies unanimously approve laws that individual members later remark as being inappropriate.
In conclusion: a crowd is always intellectually inferior to the isolated individual, yet bolder in undertaking half-witted actions. It is, in effect, a hard with the tremendous power of an enormous, stupid, and brutal beast.