In both English and Spanish, collective nouns are commonly used when referring to groups of animals, such as "a flock of sheep" (un rebaño de ovejas) and "a school of fish" (un banco de peces). But they are also used in many other contexts as well. It is common to follow a collective noun with the preposition "of" (de in Spanish) and a plural noun, as in the two examples above, but it is not necessary, especially when the meaning is clear from the context.
In standard English, collective nouns, when the subject of a sentence, are typically used with a singular verb: "The class of students studies hard." In Spanish, a verb that immediately follows a collective noun is singular: La gente tiene mucho dinero. ("The people have a lot of money." Note that this is an example of a Spanish singular noun that normally requires a plural translation in English.) But when there is a plural noun between the collective noun and verb, either a singular or plural verb can be used in everyday speech and writing, with the plural verb probably being more common. Thus you might hear both La bandada de pájaros se acercó ("The flock of birds approached," singular verb) and La bandada de pájaros se acercarón ("The flock of birds approached," plural verb), with no appreciable difference in meaning.