There may be many Pin Yin systems being used in China including Hongkong and Taiwan, but for Mandarin, there is only one widely spread. It is called Han Yu Pin Yin (汉语拼音) in ISO standard. I will talk about Han Yu Pin Yin a little bit in this post. (All "Pin Yin" below refer to "Han Yu Pin Yin")
Pin Yin is a system to help people pronounce Chinese characters. That is to say, Chinese character itself doesn't contain information of pronunciation. The mapping between the character writing and its pronunciation is "hard coded". In most case, there is no way to figure out how to pronounce a Chinese character without Pin Yin. I guess this is why Chinese language is difficult to learn.
Pin Yin is consisted of 25 English letters (except for "v") plus a special letter "ü". The letter "ü" is pronounced almost the same as the "ü" in German. In some cases, "ü" can be written as "u" but pronounces as "ü". In the input and output of Computer, sometime we use "v" to replace "ü" since the latter is not on a standard US keyboard.
The mapping from Pin Yin to Chinese character is a 1 to many mapping. It is ambiguous when Pin Yin is translated to Chinese characters, especially for names. Pin Yin is only used for learning Chinese. We don't use it in daily life. As a result, most Pin Yin you may see are names of Chinese people. When we do registration in English, we always write our names in the form of Pin Yin. Due to the high possibility of collision, you may see many Chinese people share the same name, but in fact, their names are difference in Chinese.