These are the traditional classifications of how Chinese characters were created.

象形 (xiàngxíng) - Pictographic

Characters that are stylised drawings of the objects they represent. E.g., 山 (mountain), 人 (person), 日 (sun).

指事 (zhǐshì) - Indicative / Ideographic

Characters that use abstract symbols to indicate an idea or a logical relation. E.g., 上 (up), 下 (down), 三 (three).

会意 (huìyì) - Associative Compounds

Characters formed by combining two or more pictographic or ideographic elements to suggest a new meaning. E.g., 休 (rest = 人 person + 木 tree), 明 (bright = 日 sun + 月 moon).

形声 (xíngshēng) - Phonetic-Semantic Compounds

The largest category (over 80%). Characters with one component suggesting meaning (semantic radical) and another suggesting sound (phonetic component). E.g., 妈 (mā, mother) has 女 (female) for meaning and 马 (mǎ) for sound.

转注 (zhuǎnzhù) - Mutually Explanatory

A debated category referring to characters with similar meaning and etymology that can explain each other. E.g., 老 (lǎo, old) and 考 (kǎo, aged).

假借 (jiǎjiè) - Phonetic Loan

Using an existing character purely for its sound to represent a new, often abstract, word with no original character. E.g., 来 (lái) was originally a pictogram for "wheat," but was borrowed to mean "to come."