Traditionally Chinese characters are divided into six categories (六書 liùshū "Six Writings"). This classification is often attributed to Xu Shen's second century dictionary Shuowen Jiezi, but it has been dated earlier. This page shows four of those categories.
Thought to be the oldest types of characters, pictographs were originally pictures of things. During the past 5,000 years or so they have become simplified and stylised.
Ideographs are graphical representations of abstract ideas.
Compound pictographs and ideographs combine one or more pictographs or ideographs to form new characters. Both component parts contribute to the meaning of the compound character.
The character for thought was originally a combination of the characters for brain + heart. In the modern character the brain component has been replaced by the character for field, which is very similar to the one for brain.
Semantic-phonetic compounds represent around 90% of all existing characters and consist of two parts: a semantic component or radical which hints at the meaning of the character, and a phonetic component which gives a clue to the pronunciation of the character.
Characters containing the same phonetic component may have the same sound and the same tone, the same sound but a different tone, the same initial or final sound, or a different sound and a different tone.
Phonetic components are generally a more reliable indication of pronunciation than semantic components are of meaning.