What are the major differences between Aikido and Tai Chi Chuan?

Both have a similar concept. Use your opponents force against them. So what is the major difference between the two? Is it the way it uses it's techniques? Why? There are different forms of Tai Chi Chuan, but they're all Tai Chi Chuan. What seperates the two so much?

A couple of good answers were given here, but let me throw in my two cents. While Aikido training is primarily applications based, Taijiquan is not only forms based. The forms are a very important portion of the training, but without working the applications, there will be no true understanding of the principles. For a complete level of training and to reap the full benefits, appropriate time should be spent on the basics, forms (empty hand and weapons), applications, and research of the principles.

Also while Aikido is a very effective form of martial arts (when taught properly) the techniques are generally defensive in nature. This is due to the philosophical teachings of Aikido's founder, Morihei Ueshiba. Taijiquan (in my experience, Chen style) still contains the offensive and defensive techniques that were seen on the ancient battlefields. Other arts that formed much of the foundation of Aikido like Aiki Jujutsu and Daitoryu (some sources claim that Baguazhang was another martial art that influenced Morihei Ueshiba) also still maintain battlefield efficacy.

Conceptually the two arts are very similar, and they can look very similar in application by people at the highest levels. The quality of movement and application of the best practitioners of either system can look somewhat similar, i.e. they are all using softness, turning of the waist to generate power.

Aikido is a Japanese art, and much of its philosophy is drawn from Japanese culture, Zen buddhism and shinto. Broadly speaking, its focus is more on throwing and locking. As an oversimplification, you sucker your opponent into "giving" you something to disrupt him or throw him with.

Tai Chi is Chinese, and whilst its history is very much a point of debate, its effectiveness is not. Again, it uses softness and touch sensitivity to figure out where your opponents weak points are and intelligent use of force to overcome him.

Well the biggest difference is the training methods. Aikido training (again an oversimplification) in a dojo is primarily technique based, with lots of partner work. The advanced concepts being trained on top of the basics.

Tai Chi training is based around the forms, because forms are historically a way to store information and it also enables the student to do solo practise a bit more easily. Push hands is the primary vehicle of partner work in Tai Chi.

Aikido is developed with the samurai sword in mind so the defense tends to control the opponent through their attacking arm and the redirection tends to be more sweeping to redirect the weapon as well as the attacker (but works without the weapon). The energy/power involved tends to be more kinetic(motion). Aikido will mostly train with an opponent to develop technique.
Tai chi would develop getting into your opponents space and feeling their strength and weakness while maintaining your own structure. The energy/power tends to be more potential like a spring. With a good spring structure you allow your opponent to compress your spring and return the force. Tai chi starts with the forms to develop your good structure/posture.
I'm not very experienced in either style but I would assume that the principles of both styles would complement as you develop your skills.

At the core principles.... Not much. Both actually express the concept of Yin/Yang (though they may be called by different names). Both are considered "soft" or "internal" martial arts. Both deal with how to effectively use Chi or Ki to move the body as efficiently as possible. They even share some of the same expressions of technique. The application of Needle at Sea Bottom I've seen most is a variation on Aikido's Nikyo technique.

The differences really come from the cultural backgrounds. It's in the "flavor" of the art. That and the dogma that has been propogated by followers of each art have given the appearance of more differences than similarities.

(Just a side note.... There are some scholars who believe Aikido was actually influenced by Baguazhang - Tai Chi Chuan's sister art.)

"The unnamable is the eternally real.
Naming is the origin
of all particular things."
-- Tao Te Ching, Chap. 1

aikido camed from japan

tai chi chuan came from china

a small sea