"This is a woman."
Eto is roughly "this is" and is gender neutral. "etot" is "this" and is specifically masculine. You could say for example, if you are picking up a piece of ID and saying who it represents, "eto etot chelovek" - this is this person.
If I wanted to say something about this woman, then I'd say эта женщина - эта женщина пьёт кофе, эта женщина ест хлеб, эта женщина готовит борщ. However, in this sentence it's not describing the noun. This is a woman, это -- женщина. It's on the other side of the verb - the verb to be doesn't appear here, but it's understood. So in this case, it isn't describing the noun, and so it doesn't have to agree in case/number/gender.
I can't figure out how helpful or not I'm being here, so I'm going to suggest you go look at the sticky thread at the top of the Russian forum that's all about how to use это!
I've tried zhenshina, zhensheena, xhenshina... nothing is working, and i can't get past the level...