Levels of Meditation :
When people say to me (frequently) "I can't meditate", it's based on two mis-conceptions : (a) there's only one level of meditation and (b) everyone else is meditating wonderfully and only they are not.
Level One :
I would say that there are TWO GENERAL LEVELS of meditation. Not styles, but levels. In the description of the EIGHT LIMBS OF YOGA, we see that meditation (sanskrit : DHYANA) is the seventh of these eight stages, which suggests that one should be proficient in the preceding levels. In other words, DHYANA is an advanced level which many of us may experience for very short periods or not at all.
Advanced practitioners may be able to go into DHYANA at will, and remain there as long as they wish, but this is MOST UNLIKELY for normal people, especially those who have never undergone any spiritual or ascetic training.
It is difficult for me to define this advanced level, as I am not an advanced practitioner. From my limited experience, it feels as if one has entered into a new arena, that is stable, vast, and definitely different, also, one enters here silently, without the least struggle or effort.
Level Two :
At this level anyone can meditate, you may be doing it right now.
Whenever you are completely absorbed in an activity such as reading, playing a musical instrument, cooking, practising yoga, listening to music, various drills, combat etc. then you are in a single, focused thought stream and this is meditation.
The key point is : Focus on a Single Thought Stream.
The task has to "capture" your mind entirely; if you are rock-climbing, you cannot help but focus, as even a small error can result in disaster; but it need not be such a dramatic activity, it really depends on the person. A gripping adventure film can entirely occupy the mind of one person, while another has their mind on several other things.
This reveals something very important. The person who was entirely transported by the film, and who allowed their mind (stream) to be drawn irresistably forwards; that person will emerge from the cinema feeling mentally rested and perhaps a little elated, the other will not. One was meditating, the other was not.
In reality, ANY activity may become a meditation - it depends on the mind of the person at that time. Whether the task is enormously complex (open heart surgery) or mundane ( making tea*), yet when it is undertaken with concentration, the mind will assume a special attitude, and there begins the journey.
What is the Purpose of Meditation ?
We spend most of our waking hours chasing thought streams which may leave us tired and dispirited. Thoughts may play out once, or repeat incessantly for minutes, hours, days ... years. Each time a thought is released, it has some subtle effect on ourselves, and it goes on to affect those around us and the entire universe. In addition, every thought is like a trickle of water; and repetition and intensity mark the soul proportionately.
Therefore, the first very practical purpose of Meditation is to prevent negative thought streams from taking over the mental landscape and making us unhappy.
* The Japanese Tea Ceremony is a beautiful example of a meditation for those sensitive and skilled in this spiritual art form.