Collective nouns are common nouns that designate various beings or things of the same kind, but they are designated in a singular form, as a group.

Examples of collective nouns are:

group of ovejas (sheep)
el rebaño - flock

group of soldados (soldiers)
el ejército - army

group of árboles (trees)
el bosque - forest

group of cantantes (singers)
el coro - choir

Example sentences:

¿Intentabas tocar la colmena sin llevar protección?
Were you trying to touch the hive without wearing protection?.

Hay 967 libros en esta parte de la biblioteca.
There are 967 books on this side of the library.

Los huevos cuestan un dólar la docena.
The eggs cost one dollar by the dozen.

La muchedumbre se metió en marcha hacia el palacio real.
The crowd went into the royal palace.