Could anyone tell me the difference between Qi Gong and Tai Chi?

Qigong is the coordination of different breathing patterns with various physical postures and motions of the body for health maintenance and therapeutic interventions of some ailments. Qigong relies on the traditional Chinese belief that the body has an energy field generated and maintained by the natural respiration of the body, known as Qi. Qi, pronounced Chi) means breath or to breathe. Qigong excurses are more akin to Western Calisthenics in that each exercise is performed a number of times (20, 40 or more) on both sides before the next exercise is performed.

Taijiquan (Tai Chi Chuan) means literally, "Supreme Ultimate Fist" and is a form of internal Martial art. There are many different styles of Taijiquan, although most agree they are all based on the system originally taught by the Chen family. Traditionally, Taijiquan was taught as a martial art and many traditional practitioners still teach it as one. Still, it has become popular among many people who have little to no interest in the martial application of the art. The study of Taijiquan involves three primary subjects:

Health: Similar in concept to Qigong, Taijiquan uses movement to stimulate and manipulate Qi.

Meditation: By maintaining Mindfulness during the execution of the Taijiquan patterns, a deeper spiritual experience can be obtained.

Martial art: many of the slow movements and position can be directly translated into combat postures and the philosophy of the art translates well into combat situation.

As a note, many notable Taijiquan masters will teach Qigong as well as Taijiquan, usually as preliminary or basic exercises.

I hope this answers your question.