A worker is a person who works. This usually means a person who does manual labour, like manufacturing goods. In economics there are three factors of production. These are labour (the work done by workers), land (usually people need some space to produce something), and capital (it will cost resources, or money, to produce something).
People who work, usually get paid for the work they do. This is either called wages (if it is bound to a time period, like an hour), or a salary (if it is independent of the time taken).
May Day is an international holiday for workers' rights.