Half-siblings share only one parent instead of two as full siblings do and are on average 25% related, i.e. the children that the parent and stepparent (i.e. mother and stepfather, or father and stepmother) have together. Theoretically, there is a chance that they might not be related at all though this is very rare and is due to there being a smaller possibility of inheriting the same chromosomes from the shared parent.
Half-siblings can have a wide variety of interpersonal relationships, from a bond as close as any full siblings, to total strangers. Some consider their paternal siblings their half-sibling or sibling on their father's side and some consider their maternal sibling still just their full sibling or sibling because they give more weight to a shared mother and womb, and because children are usually raised by and/or have more interaction with their mother.