Day 1

Here's a thing: the difference between practical and theoretical understanding.
If you're reading this you understand English. But that's not at all the same thing as understanding, say, physics. If you say you can understand physics, you mean you know the rules of how physics is put together. If you say you understand English, you just mean that if someone speaks to you in English, you understand them.
So linguists want to understand English like physicists understand physics. Actually, moreover they want to understand languages generally. Linguistics is (ideally) a science - it follows the scientific method of empirical enquiry and criticism to try to arrive at real knowledge. This is a relatively recent idea, maybe fifty or sixty years old, and isn't always the case, but it increasingly is.

Let's peel this project open, like an onion. Generally, each layer interacts most strongly with the next layer up, so ranking it does make sense. You'll find things often get wuzzier and less rigorous as you head outwards, as well.

At the heart, you have phonetics. This is mainly split into articulatory and acoustic phonetics - how we make sounds, and what sounds we make, respectively. These sounds are phones.
One step up, you have phonology. This is all about the underlying structure of these sounds in our heads and how we string them together, which does not necessarily correspond one-to-one with the phonetics.
Step up again and you have morphology, which is how you string whole morphemes together. A morpheme  is the basic unit of meaning - lexical morphemes like "apple" or "love" or "kick" or "sour" describe certain things in the world, and inflectional morphemes like "apple-s" or "love-er" or "kick-ed" tell us about how these relate to one another.
One step up from that is syntax - how words are assembled into sentences. Why can we say things like John ate the apple but not *John eaten the apple?
Step up again and you have semantics - I kind of lied when I said morphemes were units of meaning. Morphology doesn't care about meaning - it's more interested in how they're put together in an arbitrary kind of way. Semantics analyses literal meaning and tries to render it into formal, logical structures.
One step out and you have pragmatics, the study of speech in context and in use, which deals with non-literal meanings ("Is there going to be dinner?" "John broke his wrist." - we expect that John was going to cook dinner, so the answer is no.)
Finally, you end up in applied linguistics (or 'hyphen-linguistics'). This means anything like neurolinguistics, psycholinguistics, stylistics or sociolinguistics, all the places where linguistics meets with other fields.

Sometimes more or less layers are had - some people might group up syntax and morphology, phonology and phonetics or semantics and pragmatics, others might tease out new layers like discourse analysis from pragmatics.

We'll make one last quick distinction - in a lot of linguistics there's a tension between the generative and psychorealistic approaches. Generative approaches just want to model language; psychorealistic approaches want to understand what the brain really does. Mostly, I'll be talking about generative linguistics.

If there's any questions you want answered, feel free to comment. I'm planning on tackling each layer in order from the inside out for the next seven posts, and then answering questions or frothing about things I find fascinating.

I'll try to limit myself to 500 words or less per post ><
[edit] okay, 250 words, 500 words is more than I thought D: