How to disambiguate between various meditation practices? From what I understand, different schools of Buddhism teach different subsets of meditation techniques, or different variations of the same techniques. However, many meditation resources don't specify which technique is meant to achieve what goal, nor the source of the particular technique. There seems to be overlap and combination of techniques, e.g. observing and counting the breath in Zazen, vs. simply observing in Anapana. Some insist that you must sit a certain way, others say to just "sit comfortably." Some you must practice for two hours, morning and evening, others say 20 minutes is sufficient. Furthermore, some sources say that their meditation techniques are not compatible with others, e.g. S.N. Goenka says if you practice Vipassana, you must not practice any other technique (but, even more confusingly, he also teaches Anapana and Metta)

Could you help me understand the difference between these various techniques (how it's performed, what its goal is, advantages/disadvantages), their relationship to each other, and their origins?

Are there any important ones I'm missing from this list?


Mindfulness meditation
Zazen
Vipassana / insight meditation
Anapana
Metta / loving-kindness meditation
Tantric meditation 
Walking meditation
Mind clearing (not sure the right name for this - instead of picking an object of meditation like the breath, the goal is to achieve a state of complete thoughtlessness) 
Samatha
Transcendental meditation


(I think some of these might be special cases of others, but it's not clear to me)
I agree with Dhammadhatu in that the different meditations seem confusing when you don't know what it is all about.

When you know the underlying principle, you know how these different meditations actually approach same thing from slightly different angle, and you see how they try to explain something that is hard to explain in words, and emphasize what they think are the most important points of that.

And I'm telling you right here and now, what this principle is. It is about dukkha, origination of dukkha, cessation of dukkha, and path leading gradually to cessation of dukkha. I know this sounds so standard and abstract as to almost make no sense, but I swear this is what it is about. 

In our life with our every choice and every action we either generate more dukkha (either right now, or increase its odd in our future) or we ... well ... we simply don't create it, or at least reduce the odds of it happening.

This is what all types of Buddhist meditation about, all of them, without exception. They all are various ways to just... Well... They are all exercises at not creating dukkha. Some are more direct and some roundabout. Some more straightforward and some more implicit. But they are all about same thing, essentially. Not creating dukkha.

And how is dukkha created? Whenever in your mind there's craving for things to be other than they are, or in other words there's conflict between "is" and "should", or in other words there's discord - then there is dukkha. And correspondingly when there is no craving, no conflict, no discord - there's no dukkha. And then there's coarse craving, coarse conflict, coarse discord, coarse dukkha -- and various degrees of more subtle craving, subtle conflict, subtle discord.

So meditation is about somehow solving this problem. Removing craving, conflict, discord. Naturally, there are different ways to approach this. Some do it gradually. Some try to do all at once. Some use more force, some are more gentle.

Usually, there is understanding that craving for things to be other than they are, conflict between "is" and "should", and discord - are implicit in our frame of reference, in our evaluation, in our perspective. So most meditations involve some sort of technique that conditions (or unconditions) one's perspective, in order to change the basis for evaluation. Other techniques try to remove all evaluation altogether. In any way, it's all about dukkha and cessation of dukkha. 

If you think about it, not all methods are appropriate for all situations. If dukkha is generated by more coarse conflict, then cruder methods work better. If dukkha is generated by subtle inner discord, then very tricky techniques are required. Since most practitioners make progress starting from coarser and then to more subtle, it makes sense that different techniques are appropriate depending on where you are on your individual ladder.

However, many meditation methods are not aware of where they fall. They are often taught in isolation of the broader context, so people don't know what they are doing and why. This is a problem. 

If you understand the big picture, you can just sit down and practice. You don't need lenghthy explanations. You don't need to study all these different methods. You can just sit and watch your mind, and try to figure out the problem of dukkha all by yourself. It actually works better that way.

Just sit down and watch:  here is dukkha, this is how it feels, this is how it looks, this thing, this thought, this mindstate has an element of dukkha, and I can see it very clearly.

Then sit and try to see, this is how this dukkha originates, this is the craving for things to be different than they are that it comes from. This is the conflict between "is" and "should" that it comes from. This is discord that it comes from.

Then sit and try to do it. Here's when I let go of this craving for things to be different than they are, this dukkha no longer originates. Here's, when I reconcile the conflict between "is" and "should", here's when I remove inner discord, the dukkha ceases to arise.

Then this particular dukkha is gone, but something is still bothering us, and so we go on to the next iteration. This bothering is a piece of dukkha. I can see it very clearly.

And so on, all the way to Nirvana. It's kind of simple when you get what it is all about.

All meditations in your list, essentially do the same thing, except they don't explain it step by step like I did, instead they just say, "here is exactly how to stop dukkha: watch your breath, and if anything, including dukkha, comes up, come back to breath" (Anapana) or they would say "just label everything that comes up, without judging" (Vipassana) or they will say "just sit. there's no need to change anything, just sit" (Zazen), or they would say "sit and believe that you are a Buddhist deity, and learn to really feel it" (tantric), or they will say "sit and try to love everyone and everything (metta)", or they will say "sit and do nothing until you get lost in thought, then read mantra and come back to doing nothing" (transcendental), or they will say "walk and pay attention to every tiny detail of your walking muscles" and so on and so forth. But now you get it - all of these are just their tricks to stop making dukkha. When you know what it is about, you don't have to blindly follow any of that. You can sit down and practice your own real Buddhist meditation, in accordance with Noble Truths.