High level, it's about how one person puts across their message to another, and how that message is received.
1) how you communicate
By this, I'm talking about your bread and butter presentation skills, how loud you are, how your energy is, how you claim space and how you project. I could go on, but hopefully you get the message...
2) what you communicate
Here I'm talking about your content, and underpinning that is your key message (more of which later). This is how you break down what you're going to say
3) who you are communicating with
Who are you talking to? If it's a business presentation, can you find out who, or are you fortunate to already know some of them. If its over the course of a gig, can you at least come up with some personas of who is likely to be out there, and why they're stood listening to you?
4) how they will receive it
A little more complicated than the above, but understanding audience behaviours. If we take TED as an example they make speakers limit themselves to around 10minutes. As that means the audience won't get too bored and start thinking about food, sex and their lives.
5) how you both leave the situation, and what you take with you
This is the big one... what is the message you want to ensure is taken away, as well as how you want to feel having completed your communication. A lot of my clients express relief, but I'm not a fan of that... if you get 1-4 right, then something more pleasurable should be attainable. The idea of a key message may be really impactful (think 'I have a dream'), or it could be slightly more abstract - with artists I've found it could be coming across as funny.
Either way, this informs the other four and how you do all your communication with your audience.
You may be asking why it's last if it's the most important. As I want it to be your takeaway!