The answer shown above is good. In addition, to give you a possibly better perspective: This is an apple = Esta es una manzana. It is an apple = Es una manzana. In English, if you are showing a toddler what an apple is, and you point to an apple, it sounds better to say "This is an apple", igual como en espanol suena un poco mejor a decir "Esta es una manzana". In either case, both are grammatically correct!

They are used differently. If you are introducing someone who is your friend to another person, you might say, "This is my friend Sam". If you are in a room with lots of people and someone asks, 'who is that person over there with the red hair', you could answer, 'it is my friend'. If you are showing someone a fruit who doesn't know what it is, you will say 'this is an apple'. If someone picks up an apple and says 'whose apple is this', you could answer 'it is my apple'.