辛苦 describes work or effort that is exhausting and hard, while 累 (lèi) refers to the resulting feeling of tiredness in a person. Compared to 艰苦 (jiānkǔ), which emphasizes harsh conditions or a difficult environment, 辛苦 focuses on the effort and trouble involved. As a verb, it is used in polite expressions like 辛苦了 to thank someone for their hard work.