"Это" - when you point at something to define "what is it?", no matter if it is one object or a set of objects. "Эти" - its when you identifiying it by question "which one(s)" - i.e. these ones Examples: 1) Imagine a man pointing with a finger to tomato and then to a set of apples. He will say: Это помидор, это яблоки - this is tomato, this is apples. 2) When he wants someone to give him these apples he will use: дайте мне эти яблоки - give me these apples.