Use warm (105 - 115 degrees Fahrenheit), but not hot water if making a preferment, starter, "biga" or "poolish", i.e. a combination of water, yeast and flour and sometimes sugar. You shouldn't go much over 115 degrees because yeast dies at around 130-140.
Most of us find 120 degree water scalding hot. If the water is too hot to handle, it could kill your yeast.
Don't be a chicken about warm water though. If it isn't warm enough, you won't stimulate the leavening process -- you won't create the by-product carbon dioxide and get those lovely bubbles that make the crumb (texture) so wonderful.
"Oven spring" is another crucial part of temperature control. For most yeast breads, your dough should be greeted by a 500 degree oven. This will create a rapid puffing up of the loaf before the crust forms and locks out further expansion. After five minutes you should turn down the oven.