备份 (bèi fèn) specifically refers to backing up digital data — it's both a noun ('a backup') and a verb ('to back up'). It differs from 后备 (hòu bèi), which means 'backup' in a broader sense (e.g., reserve personnel or supplies), and from 备用 (bèi yòng), which means 'spare' or 'reserve' (e.g., a spare tire). Use 备份 only for computer files, databases, or other electronic data.