Hydrogen sulfide is the gas behind a rotten egg smell.
What it is
Hydrogen sulfide is a dissolved gas, produced either by sulfur bacteria in a well or inside a water heater by the reaction between a magnesium anode rod, sulfate, and bacteria in the warm tank. It is measured in milligrams per liter, and people can usually smell it well below the level at which it does anything else.
What it usually does
The smell is the main complaint. It also tarnishes silverware, darkens brass and copper, and corrodes metal parts over time. Because it is a gas, it comes out of solution in the shower, which is usually why a bathroom smells stronger than a glass of the same water.
What it does not prove
A smell does not establish a level, and a level does not establish the source. Sulfate in the water is not the same thing as sulfide, and sulfur bacteria are a separate test again. The most useful free check is which tap smells: hot only usually points at the heater, both taps usually points at the supply.
When to test
Usually worth measuring before sizing anything, because sulfide treatment is chosen from a number and the nose does not produce one. Sulfide also escapes from a sample bottle quickly, so it usually has to be field-tested or preserved at the time of collection rather than mailed in casually.
See what this usually means for your water.
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