A checklist is a written list used to verify that tasks or items have been completed or accounted for — the emphasis is on checking off what is done. A to-do list, by contrast, is for planning future actions and does not necessarily imply verification. An inventory is a detailed record of goods or items in stock, often used in retail or warehousing, and its purpose is counting rather than checking tasks off. A schedule arranges items by time and is used for planning when things will happen, whereas a checklist arranges items by completion status. For example, a pilot runs through a pre-flight checklist to confirm every system is working, keeps a to-do list of meetings this week, checks an inventory of spare parts, and consults a schedule of departure times.
Checklist is a compound formed in the early 20th century from 'check' (to verify or examine, from Old French eschec, meaning a check at chess) and 'list' (a written series of items, from Old English liste, meaning a strip or border). The combined sense of a written list used for verification emerged in aviation and industrial contexts before spreading to general use.