Articulatory phonetics deals with how the human vocal tract creates sounds.
Knowing the principles of how the vocal tract works can help science fiction and fantasy writers to create languages that follow naturalistic patterns of pronunciation, thus making created languages that seem more natural.
One of the key assumptions in the following discussion is that we're working with a species which, like humans, can perceive vibrations in the air (whether through ears and hearing or by other means like antennae). While this does restrict us somewhat, it still allows for a lot of possibilities.
Let me begin with a caveat before we begin our tour of the vocal tract. If you've never studied linguistics, this may appear complex - but it's not as bad as it seems. Just because there are a lot of variables you can change about a language doesn't mean you should go about trying to change them all.
Okay, so here we go:
1. Powered by the diaphragm, the lungs emit an airstream that can be shaped by other parts of the vocal tract. This is the power source for the sounds. Change this element, and you'll have a drastically different language, but one that will be a bear to transcribe into English!
2. The vocal cords can vibrate when the airstream passes by them. All vowels are "voiced" sounds, i.e. sounds with where the vocal cords vibrate. So are consonants like b, d, z, v, y, l, r, n, and m. In the case of an alien, it's important to know that this creature possesses something like vocal cords, or at least something able to create a consistent humming vibration, if you're going to use any voiced consonants in transcribing its language. Language sounds without this vibration are called "unvoiced." Whispering is entirely unvoiced.
3. The mouth is a resonating space for vibrating air. In human languages, the quality of vowels is altered when the tongue is used to alter the shape of the mouth space. The position of the tongue is described in two dimensions: height (high, mid, low) and front/back. Here are some examples of the position of English vowels.
[i] as in "feet"=high front [u] as in "hoot"= high back
[E] as in "bet"= mid front [o] as in "boat"= mid back
[ae] as in "hat"=low front
If you go into any beginning linguistics textbook, it's easy to find a graph of the mouth space and the vowels involved; you can also Google "vocal tract." Here, I'd prefer to talk about what to do with them. If you have an alien, try to think about the kind of resonating space it uses to create speech sounds - the length of its muzzle or other factors might change things significantly. You can also think about how it might change the shape of that resonating space (with tongue or other muscles), because this would affect its ability to pronounce human languages.
If you have a human or fantasy human, the problem is easier, but you can still think about how the language pattern might use vowels with different characteristics. Do your people generally avoid mid vowels? Avoid back vowels? Do they tend to pronounce vowels across a word with the same kind of mouth and tongue position (say, making all vowels in a single word high)? Do they generally keep their lips rounded or unrounded? There are lots of options here.
4. The air flow can be stopped or blocked in different ways by the tongue, teeth, and lips. When the air flow is blocked completely, that's called a stop (for example, p/t/k/b/d/g). When it's still flowing but partially obstructed, that can be called a liquid (l/r), an affricate (ch/ts) or a fricative (s/th/f/z/v). W and y are called glides. Consider the "tools" your creature or person has for altering air flow. Where will most of the obstructions occur? Far back in the mouth near the uvula, as with French R? In the front with the lips? At the alveolar ridge behind the teeth where we create sounds like t/d/s/z?
There's more I could talk about, but I don't want to go overboard...
In fact, you'd be surprised how few things you need to change to give an entirely different flavor to the alien words you use. Here is an example of a language that I recently created.
I had an alien with a long muzzle and tongue, so I decided that there were a lot of different kinds of "l" and "r" sounds in this language. In English I decided to use single "l" versus double "ll" and single "r" versus double "rr" to indicate these sounds, even though I didn't know exactly what they sounded like. I also decided to avoid all unvoiced consonants - mostly for the sake of argument, and for giving the language a distinctive "feel." That means plenty of m/n/d/g/b/v, etc., but no p/t/k/s/f.
To my mind, the biggest advantage of using principles of articulatory phonetics is this: if you use natural language patterns to guide your choices, the resulting created languages will seem less arbitrary and more convincing.