Although the word labor seems to suggest tough, physical work that makes you sweat, any type of work, whether physical or mental, can be considered labor.
Labor is usually associated with very hard, physical work, suggesting fatigue and aching muscles. In fact, a woman giving birth is said to be in labor, giving true meaning to the phrase "labor of love." The word can also be used as a verb, as in "He had to labor many years to pay off his debt." That sense of hard work fits in with the Labor Party of some nations, a party generally favored by and supporting the working class.