«Это» has 2 meanings. 1 – It might be a demonstrative pronoun to the neuter (я купил ЭТО масло (I bought THIS butter), я хочу, чтобы ты увидел это чудо (I want you to see THIS wonder.) 2 – it might be a subject in the sentences like Это – масло (it is a butter), Это чудо, что я тебя встретил (It’s a wonder, that I’ve met you). «Эта» has only one meaning as a demonstrative pronoun to the feminine gender. Мне нравится ЭТА машина (I like THIS car), ЭТА кошка не есть рыбу (THIS cat doesn’t eat fish). You’re right these 2 words are pronounced almost similar in the fast talk anyway they are not the same.

этот мужчина - this man (masculine) эта женщина - this woman (feminine) это животное - this animal (neuter gender) (the word "это" is adjective or pronoun, it seems it's pronoun rather than adjective ) эти люди - these people (plural) ---------- это - it (the word "это" is noun)

I'm Russian and I dare to say that most often they are pronounced the same in ordinary speech. Though sometimes этО could be said with sound O like in mOre (rare)

to my mind they are said differently look this lesson : http://www.youtube.com/watch?v=4jphbTSAYgA