Green or Brown

Looking out of the window at our garden this week, it looks a sorry state, the lawn is brown instead of green. Yet looking closer I could see some bits of green. On closer investigation - going outside - walking over the turf the ground felt as hard as iron, not a bit springy how grass should - I found that the areas of green I could see, weren`t grass at all, they were weeds. So, how is it that although the grass in the heat goes brown the weeds don`t?! Is there no way of killing them off.