A collective noun is singular, if the entities forming the group are acting in unison.
A collective noun is plural, if the entities forming the group are not acting in unison, rather acting individually.
Countable Noun- Entities which we can count.
Uncountable Noun- Entities which we can't count.
Take "team" for example which consists of a number of players.
Every afternoon the baseball team follows its coach out to the hot field for practice.
Team = singular; follows = a singular verb; its = a singular pronoun. All members of the team arrive at the same place at the same time.
After the three-hour practice under the brutal sun, the team shower, change into their street clothes, and head to their air-conditioned homes.
Team = plural; shower, change, head = plural verbs; their = a plural pronoun. The teammates are dressing into their individual outfits and leaving in different directions for their individual homes.
Similarly, a jury of judges.
When they agree, we say:
The jury agrees on the verdict.
But when they diagree, we say:
The jury do not agree on the verdict.
As for people, I guess the other answers have already clarified the issue. So I am not addressing it.
Also collective nouns are usually singular. They have separate plural forms.
Collective nouns are names of collections or groups that can be considered as individual units. Army, family, flock, committee, and herd are all examples of collective nouns. Collective nouns are usually countable nouns (e.g. two armies, three families, six flocks, etc.) but are often confused with mass (or uncountable) nouns. The confusion generally stems from misuse of the term mass noun to mean collective noun.
UPDATE- This is relevant to mention that later Snailboat pointed out that this rule which I have given in the answer is a more UK usage than the US. Americans are most likely to consider any collective noun as singular in most of the case.
Due to an innovation in British English (BrE), collective nouns can take either singular (formal agreement) or plural (notional agreement) verb forms, according to whether the emphasis is on the body as a whole or on the individual members respectively; compare a committee was appointed with the committee were unable to agree. The term the Government always takes a plural verb in British civil service convention, perhaps to emphasize the principle of cabinet collective responsibility. Compare also the following lines of Elvis Costello's song "Oliver's Army": Oliver's Army are on their way / Oliver's Army is here to stay. Some of these nouns, for example staff, actually combine with plural verbs most of the time.
In American English (AmE), collective nouns are almost always singular in construction: the committee was unable to agree. However, when a speaker wishes to emphasize that the individuals are acting separately, a plural pronoun may be employed with a singular or plural verb: the team takes their seats, rather than the team takes its seats. However, such a sentence would most likely be recast as the team members take their seats. Despite exceptions such as usage in The New York Times, the names of sports teams are usually treated as plurals even if the form of the name is singular.
The difference occurs for all nouns of multitude, both general terms such as team and company and proper nouns (for example where a place name is used to refer to a sports team). For instance,
BrE: The Clash are a well-known band; AmE: The Clash is a well-known band.
BrE: Spain are the champions; AmE: Spain is the champion.