NOTE: This page is far from complete!!
Phone-based input methods utilize the romanization of the sound of Chinese characters. In most cases, there are three parts; initials, finals, and tones.
For example, if Pinyin romanization system is used, the romanization of "中" is "zhong1". "zh" is the initial, "ong" is the final and "1" is the tone. "終", "鐘" and some other characters' romanization is also the same because they are homophones. Technically speaking, there are many-to-one mappings between a text string and a Chinese character.
There are many systems exist. Check the following page to get a big picture.
Major romanization systems used in daily life as well as input methods are Pinyin and Zhuyin.
Some people also use input methods based on romanization of a dialect of Chinese, say Cantonese.
The Pinyin system itself is standard and fixed. However, using it directly is not very effective. There are many many ways to improve the performance. Before I can document them all, you may check the following page first: