Simply an open top container or tube, calibrated in inches to measure rainfall. They need to be hand-emptied after each rainfall to ensure correct measurements.
Rainfall amount is described as the depth of water reaching the ground, typically in inches or millimetres (25 mm equals one inch). An inch of rain is exactly that, water that is one inch deep. One inch of rainfall equals 4.7 gallons of water per square yard or 22,650 gallons of water per acre!