That depends on what definition of "work" you are using. The most fundamental equation in thermodynamics is deltaU = q + w, where q is heat and w is work. However, some textbooks say q - w, where w is work done BY the gas (or system). You then have to switch all the sign conventions you use. So here's what I'll say...
When work is done ON a system, the energy of the system increases. When work is done BY the system, the energy of the system decreases. When heat flows into a system, its energy increases. When heat flows out, its energy decreases. How you define POSITIVE and NEGATIVE depends on how you're looking at the problem (i.e. from the system's point of view? work done by or on the system?). One obvious thing to keep it in mind is that positive and negative are the opposites of one another. For example, POSITIVE work done BY the system is the same thing as NEGATIVE work done ON the system. (double negative cancels out) Confused? I was too, but once you get this critical concept, thermodynamics on the MCAT is incredibly easy.
