Everything written above is correct. Especially I liked the link, it provides a very detailed answer. I would just add some simple explanation of my own
OK, there are two "это"s: "это" used as a noun, and "это" used as an adjective. Both of them have the same meaning ("this"), but they differ in their syntactic usage.

When "это" substitutes a noun, it is used independently. In this case, "это" is always neuter and singular. It is often called "unchangeable это", but I would say it is not an accurate statement. "Это"-noun never changes its gender and number, but it does change cases.

Consider the following examples:
Это стол. (Стол is masculine!) – This/that/it is a table.
Это книга. (Книга is feminine!) – This/that/it is a book.
Это окно. (Окно is neuter!) – This/that/it is a window.
Это столы. (Noun in plural!) – These/Those/they are tables.
Это книги. (Noun in plural!) – These/Those/they are books.
Это окна. (Noun in plural!) – These/Those/they are windows.

All the examples above are in Nominative.

However, "это"-noun can be used in any other grammar case:
От этого зависит наше будущее. (Это in Genitive) – Our future depends on this/that/it.
К этому надо привыкнуть. (Это in Dative) – One has to get used to this/that/it.
Я это знаю. (Это in Accusative – the same form as the Nominative) – I know this/that/it.
Я не умею этим пользоваться. (Это in Instrumental) – I do not know how to use this/that/it.
Я ему об этом уже рассказал. (Это in Prepositional) – I have already told him about this/that/it.

When "это" is used to describe a noun, it behaves like an adjective, and it is not independent (makes no sense without a noun it modifies). In this case, "это" agrees with the noun in gender, number and case.
Compare below:
Это стол. (Стол is masculine!) – This/that/it is a table.
but:
Этот стол большой. – This table is big.

Это книга. (Книга is feminine!) – This/that/it is a book.
but
Эта книга большая. – This book is big.

Это окно. (Окно is neuter!) – This/that/it is a window.
but
Это окно большое. – This window is big.
Note: here the both forms are the same, because of the neuter gender of "окно".

Это столы. (Noun in plural!) – These/Those/they are tables.
but
Эти столы большие. – These tables are big.
etc. (all the examples above are in Nominative).

Now with the "oblique" cases (other than Nominative):

Genitive: Я не могу без этого стола. Я не могу без этой книги. Я не могу без этого окна. Я не могу без этих вещей. – I cannot do without this table/this book/this window/these things.
Dative: Подойди к этому столу. Подойди к этой книге. Подойди к этому окну. Подойди к этим вещам. – Come closer to …
Accusative: Я вижу этот стол. Я вижу эту книгу. Я вижу это окно. Я вижу эти вещи. – I see …
Instrumental: Я доволен этим столом. Я доволен этой книгой. Я доволен этим окном. Я доволен этими вещами. – I am satisfied with …
Prepositional: Я говорю об этом столе. Я говорю об этой книге. Я говорю об этом окне. Я говорю об этих вещах. – I speak about …

It's interesting to note that some languages even have completely different words for "this"-noun and "this"-adjective. E.g., in Japanese "this"-noun is KORE / SORE / ARE (depending on whether it is close to the speaker, or close to the listener, or far away from both). But "this"-adjective is KONO / SONO / ANO.