A "chassis" fan is a case fan, and that's what is meant to be plugged into the "chassis" fan connector (although case fans may also so have the larger Molex type connectors that you connect directly to the PSU, instead of the mobo).
"Chassis" is another term (usually used by geeks) for a PC case.
"Chassis Fan" is a fan that mounts on a PC case's carcass: either from inside at the front, on the door, at the back (exhaust of the hot air), at the top, or (very rarely) to the bottom or the back wall of the case.
"Chassis Fan Connector" is a pin power connector that goes from your motherboard to the case's fan, or from "Fan Controller" (Also called "Reobass") hardware to the fan.
There are two types of pin connectors: 3-pin and 4-pin.
3-pin is a standard connector, you can't control fan's RPM ("Rotations Per Minute" - speed of the fan rotation) with a 3-pin connector, but you can lower your 3-pin fan's RPM with a special additional adapter called "Resistor".
4-pin connector has a build-in fan controller (4-th pin) that allows you to control the RPM of a connected fan through the software programs on your computer ("SpeedFan" program, for example).
These days, almost 99% of modern motherboards have a 4-pin "CPU Cooler Fan Connector" by default, thus you can control the speed of your CPU Cooler Fan/Fans right from the beginning. Most of the case fans are made with 3-pin connectors, but some top tier models have either "3-pin to 4-pin" adapter, or simply 4-pin connectors by default.