IAT is Intake Air Temperature.
All these sensor have standing (Key off) resistance that's an acceptable range.
Between X ~ohms and Y ~ohms. If you get an out of range code you get your multimeter out and measure that resistance to make sure that particular component is bad and not being thrown off by something mechanical. For instance, A bad FPR will make the motor run rich, mess with the O2 sensor which tells the MAF bad info so you codes for both the MAF and the O2 sensor. But testing those sensors tells you they are good, i.e. with in range. So you don't replace them you look for the cause.
Anyway, The ECT tells the computer how warm the engine is or is not.The computer then adjusts the fuel to run richer when cold, leaner when hot. I think it's bugger to test. For each 20* increase in temp, you test the resistance. There's chart around her somewhere that gives the values. I think lots of people just test it at room temp and 190*, the running temp. Others just replace that one since it only talks to the computer and it's a 4.99 to 12.99 item.
I'm guessing that the IAT is about the same deal since it also would measure temp.