Chinese characters are not entirely unique, but share common traits with other characters.
The Hanyu Shuiping Kaoshi gives lists of characters needed to be learned for each level's written test.
The simple strokes of a simple character stands apart with a distinct meaning.
That same simple character could be included alongside other unique strokes. It conveys meaning or pronunciation to those unique strokes.
Sometimes these re-usable strokes are not themselves a character, but instead they are found in various characters.
Whether the strokes could be a separate character or not, such an item is called a "radical". Radical meaning "of or pertaining to the root", is at the core of all Chinese characters. They are indicators of meaning or pronunciation and are the primary tool for deciphering an unknown character enough to look it up in a dictionary.