"What is it?"

There are many types of meditation which frankly don't have all that much in common. In buddhist meditation, which is probably the most common type in the US, there are two main types:

- Concentration meditation (shamatha): This involves training the mind to concentrate, often by focusing on a mantra, an object (like a candle), or some imaginary object (like a mandala). There are different mental states that you can reach as you get better, called jhanas.

- The second type of meditation is insight meditation, also called vipassana. In this type of meditation you focus on the sensations of the body. It basically involves starting at your head and noting every physical sensation (warm, cold, clothing touching your skin, etc.) and working all the way down to your toes, and then going up. As you progess you begin to notice more sensations that the buddhist mental map describes as being part of the 'subtle body'. These involve things like vibrations, electric shocks, etc. You also begin to feel waves of energy traveling up and down your body. To get to this point takes 3 or 4 days, assuming you are at a retreat where you are doing it 10 hours a day. The next step is where you can feel 'inside' of yourself, like all of your organs, and then being able to feel inside your organs until they dissolve into what feels like rising bubbles. This takes a bit longer, but you can often begin to feel these 'bubbles' after a week or so. Note that you don't have to believe in any of this, it just happens whether you want it to or not. There are many more mental states and experiences you will have if you continue. There is a description here of the rest of the states:

http://kennethfolkdharma.wetpaint.com/page/Collection+of+tip...

They culminate in something called stream entry, which is the first of four stages of enlightenment according to the buddhist model.

There is also a book called Mastering The Core Teachings Of The Buddha which also has a description of all the things you go through between being a beginner and reaching enlightenment. The best way to learn though is probably to do a meditation retreat, there are free vipassana retreats offered by dhamma.org that are pretty good. (There is a suggested donation of about $300 to pay for your food and housing for the 10 days, but it's optional.) There are many reviews of this online. There are also many other retreats, but most of them cost money. One example of a popular one is the insight meditation society, http://www.dharma.org/.

There are many books also. The best one for learning vipassana is probably Mindfulness In Plain English, which you can download for free as a PDF: http://www.urbandharma.org/udharma4/mpe.html

While this is good to read, you really don't want to try it without a teacher, as you are likely to hurt yourself or do it wrong (and not make progress) unless you have an actual teacher.

The third type of meditation you hear a lot about is transcendent meditation. This is basically a form of concentration meditation, although its adherents claim it is different based on brain scans. It costs a lot of money to learn, and many consider it to be a cult. I don't know a lot about it personally.

Many other religions have some form of meditation, such as catholicism. Tibetan buddhism also has its own form of meditation, as does hinduism. (Some types of yoga are basically an active form of meditation.)

If you're looking to learn one type then vipassana is probably the place to start. Advantages are:

- Free instruction, and a large community that can help you.

- A large encyclopedia of the different mental states you will encounter, so you know what you're experiencing and whether or not they are 'part of the path' or just a distraction.

- A well defined path to follow to get from a complete beginner to very advanced.

- What r/meditation is most familiar with, which is a great place to ask questions and such.

Tibetan buddism also has a very well defined path, but it's really only taught to people within the tradition, so I can't comment much on it. It's also generally hard to learn unless you happen to live in Ithaca, NY or Boulder, CO, as there aren't many tibetan llamaseries around.

Lastly, I should mention there is also zen meditation, which is fairly popular in the US. Zen is a lot of judaism, in that it combines certain practices with what are basically brain teasers as a way of trying to coax you toward enlightenment faster. There are all sorts of parables, commentaries, commentaries on the commentaries, etc. I don't know a ton about this, but there are all sorts of blogs and places you can learn. Anyway I'm not an expert by any means, but hopefully that helps a bit.