Consider the following:
Вот пара предложений, которые я выписал в тетрадь. Это <есть> упражнения. (They are exercises)
Вот пара предложений, которые я выписал в тетрадь. Они <есть> короткие. (They are short)
Although the personal pronoun они is perfectly correct in the latter case, you will predominantly encounter this usage where a quality (or an action) of the subject for which the pronoun substitutes is described. The former case, on the other hand, tends to be used where the substituted subject is (re)defined. This usage of это is so widespread that this demonstrative pronoun is said to not only reply to the questions Какое? (where it is translated as this) but also to the question Что это? (where it then means it (is)).
In other words, in constructs like это упражнения the pronoun это serves as a personal pronoun rather than a demonstrative one. As has already been said, in its personal pronoun incarnation это is much more often substituted for inanimate, rather than animate subjects and its plural form coincides with its singular one.
Видишь этих людей? Они инженеры. - Do you see these people? They are engineers.
(Это инженеры would be also perfectly correct, however!)
Видишь эти предметы? Это стулья. - Do you see these objects? They are chairs.
(Они стулья would sound terribly weird as if someone cursed people into the shape of chairs)
I hope this helps.