I've recently been troubled by this concept of labour and labour-power. As I've come to understand, labour-power is one's capacity to work, quantitatively measured by time. Labour is, on the other hand, is the physical act of working itself. However, I wonder if you could explain how labour and labour-power are exchanged as commodities and the significance of the differentiation of such concept.
Labor is not a commodity in the Marxian theoretical system, while labor power is. Labor is something humans have always done to produce the goods and services upon which life depends. Labor power is something very historically specific, is often absent in long periods of human history, and reaches its fullest presence in capitalism. Labor power is what a worker sells to an employer. The employer "consumes" the purchased labor power by setting its owner - the worker - to perform the laboring activity (to do labor) alongside/with tools, equipment, and raw materials. The worker is paid a wage in exchange for providing to the employer what he/she owns, namely his/her labor power for a set period of time. The goal of the employer in consuming the labor power he/she has purchased is the commodity emerging at the end of the process of production, a commodity whose value (socially neccesary labor time) exceeds the combined value of (1) the used up tools, equipment and raw materials, plus (2) the value of the wage paid for the purchased labor power. That excess, surplus value, is the driving objective of a capitalist employer.
I hope this answers your question.