"This is a woman."
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!