A poolish is normally made at 100% hydration. The amount of yeast is normally very small. The actual amount of yeast is changed depending on the temperature where the poolish will be allowed to rise and how soon the poolish will be used.
This technique was developed in Poland. The Polish people introduced it to the Austrians, who in turn introduced it to the French. You can still see bakeries in France advertising "Vienna Bread" named for the process the Austrians introduced them to. Legend has it the French call it "Poolish" because they'd rather not admit they learned anything from the Polish, even second or third hand.