当中（dāngzhōng） means "among," or more literally "in the middle of." In English this is a bit like "of," where you would say, "of all of my friends, you are the most loyal." Also, 当中 can be used to express being in the middle of an activity or process that is fairly formal in nature e.g being in the middle of negotiating rather than being in the middle of talking on the phone.
When used to express "among" 当中 is a little like 其中. However, 其中 means something more like "among the aforementioned," where as 当中 simply means "among." Furthermore 其中 cannot be used in relation to processes.