Questions about radicals of Chinese characters. Radicals are dictionary headers. For questions about character components, use the tag [character-components] instead.
Radicals, in Chinese 部首 (literally "section header") are components of Chinese characters used to index dictionary entries.
They are not necessarily the semantic component in phono-semantic compounds.
For example the radical of 到 (dào “to arrive”) is刂 from 刀 dāo, which is the phonetic component, whereas the semantic component is 至.
The most well-known list of radicals are the Kangxi radicals. See this answer for more information.
Use this tag for questions about radicals, as dictionary section headers. For questions about character components use character-components. Unfortunately many old questions on this site use these tags somewhat inconsistently, as there is widespread confusion about what radicals actually are.