Can somebody please explain to me what Tai Chi is all about?

I don't really understand the concepts.

^ the answers above is correct and incorrect.

It has become this exercise that is very low impact and good for the old agers. but this was the side benefit of TaiChi

TaiChi is originally a martial arts form that places heavy importance on using your weight into your strike, using sensitivity to deflect and read movements before they happen, and turn your body soft so that any strike that does come through can not reach your center to really harm you. (the last concept is easy to understand if you think of a punch to the face and the face turns so that the impact doesn't reach the main mass) the relaxation that everyone seeks is a side effect of trying to keep your body soft as if it's hard it will take the blunt of the force.

The form practice is done slow so that you can observe every little mistake that you make in the technique and it also strengthens your legs, which is necessary to strike with all your weight and turning your body soft. The push hands are a practice to learn to listen to the other person not through eyes or ears, but through your touch. By getting good at this you can fight people without looking and you could also ready what they intend to do as they are thinking it.

Tai Chi is "Yoga in Motion". It is movement that works your core muscles and balance. It may have some martial arts movements, but it isn't really used for self-defense; more for your own chi (energy) so you can stay disciplined and self-aware, as well as in decent shape regarding your balance and inner strength. However, it is described as a meditative exercise for relaxation, health, and self-defense. Maybe check out one of hundreds of websites. Here's one:

Tai Chi as a martial art (Tai Chi Chuan) is about fluidity of motion.

One does not use force, you merely REDIRECT the force of an opponent.

Tai Chi is about:
Balance,
Rooting,
Minimizing your own muscular tension,
Not overextending any part of yourself,
Keeping your center of gravity squarely between your feet.

If your opponent applies force, you move in such a fashion that gives him/her nothing to push against. This will cause the opponent to overextend, at which point you merely "help them along."

"Correct T'ai Chi Ch'uan does not pit force against force. One learns to redirect the attacker's force back to the attacker, using as light a touch as possible. Students also learn to develop a stable center of balance, quick reflexes, and the ability to neutralize aggression without causing harm to the aggressor."
http://www.rtccc.com/about

EDIT: Not all Tai Chi forms are done slowly.

David N is very close. The main thing is to learn how to develop your Chi,if your Chi is flowing correctly through your body,the better you feel,calming,relaxing,makes meditation much easier.

There are several answers I agree with the majority of. The main principles are:
Solid rooting but not equally weighted on both legs
The waist is the comander
When one part moves, everything moves
Fangsong (loose and unclenched)
No hollows, no protrusions (no gaps in defense and attack)
The mind intent leads the qi (chi)
Steel hidden in cotton
Verticality

Tai chi (taiji, taijichuan, tai chi chuan) is a martial art, in fact probably the most powerful martial art. It also takes the longest to learn. The forms are mostly done slowly because they are done at the speed of an in or out breath per movement. Slower breaths with the mind leading the qu moves a greater volume of qi and develops better control.

A full system of tai chi, such as the Yangjia Michuan, that I practice, will have basic exercises, forms, emptyhand applications, san shou (freestyle weaponless fighting), push hands, weapons forms, weapons martial applications and freeplay. Our system has jian, dao and spear. Much of my site is about taiji jian www.chinese-swords-guide.com

I know a lot of people only practice forms and exercises and some people thing push hands is as martial as it gets, but this is diluted from the original martial art. To get the best health benefits, you must practice mindfully, and since each movement has martial applications, you must know them to practice mindfully. I'm not saying old people should fight but everyone, young and old should know what they are really doing while practising tai chi.

From what I heard its Like yoga you chill and relax let away some steam and stress I also heard it has some great punches, haha