While we have certainly seen numerous other applications of what we were learning as went along, one of the most important applications of transport (for Chemical Engineers, at least) is the study/design/analysis of heat exchangers.
In general, a heat exchanger is a device that takes heat from one fluid stream and moves it to another. One example might be the "radiator" in your apartment. Hot water (steam) enters one side, by convection to the metal pipe, conduction through the pipe, and then convection from the pipe, the hot stream warms the exterior air and ultimately leaves as cooler water (or condensed, hot water).
As there is a very wide variety of heat exchanger designs, there is also a wide variety of ways to categorize them. For our purposes, we will be investigating heat exchangers (like the one above) where the two streams are not in direct contact (you should realize, however, that sometimes the streams are in direct contact in other types of devices). So, our heat exchangers typically have some solid surface between the two fluids. The fluids then can run in the same direction (co-current), in opposite directions (counter-current); they can pass each other once (single-pass), they can pass each other multiple times (multi-pass); both fluid streams can change temperature ("two-stream"), or only one of them might change temperature ("single-stream"). (This seems to be enough complication without considering the cases where the fluids actually are in direct contact!)
Some examples of these types of devices include: double-pipe, shell-and-tube, cross-flow, etc. Many of these can also be finned and/or baffled to increase surface area and/or fluid mixing.
Distinguish between exchanger types.