What is a ecu upgrade?

I am not an engineer, but was frustrated by no answer to this question, and am readily open to being corrected. A little research has led to the following answer. Caveat Emptor.

ECU = Engine Control Unit, an electronic component of modern engine management, tweaking performance according to a series of variables which can boost power, for instance, among other things. Basically a small computer with precise instructions on how an engine should run, and which can receive different sets of instructions, as if you went from Windows XP to Windows Vista, or from Mac Tiger to Mac Leopard, for instance, to use an office computer analogy, and then all kinds of changes appear on the screen and behind it.

So an upgrade to an ECU will affect the vital statistics of an engine in different ways, depending on the intention of the "upgrade". I surmise it is often used to boost engine power beyond what the manufacturer intended, at the price of higher gas consumption, faster engine wear, and generally higher risk to the driver from potential loss of control (those infamous "single vehicle accidents") due to the rest of the car not being designed for the real world consequences of that extra power, i.e., higher speed vs. imprecise steering, suspension too soft, brakes not adequate, tires not 'sticky' enough, etc.) Very attractive however when you think that by simply downloading some software into your car's computer it becomes more powerful. Amazing when you come to think of it.