The surface pressure of Venus' atmosphere is 90 times higher than the pressure of Earth's atmosphere, as a result of the large amount of carbon dioxide in the former. Throughout Earth's history, carbon dioxide on Earth has mixed with rain to dissolve rocks; the dissolved rock and carbon dioxide eventually flow into the oceans, where they precipitate to form new terrestrial rocks, often with the help of life-forms. If this carbon dioxide were released from the Earth's rocks, along with other carbon dioxide trapped in seawater, our atmosphere would become as dense and have as high a pressure as that of Venus. Venus, slightly closer to the Sun than Earth and thus hotter, had no oceans in which the carbon dioxide could dissolve or life to help take up the carbon.
Also, Venus has probably lost almost all the water it ever had. Since Venus is closer to the Sun than the Earth is, its lower atmosphere was hotter even early on. The result was that more water vapor went into its upper atmosphere, where solar ultraviolet rays broke it up into hydrogen and oxygen. The hydrogen, a light gas, escaped easily; the oxygen has combined with other gasses or with iron on Venus' surface.
Studies from the Earth show that the clouds on Venus are primarily composed of droplets of sulfuric acid, with water droplets mixed in. Sulfuric acid may sound strange as a cloud constituent, but the Earth too has a significant layer of sulfuric acid droplets in its stratosphere. However, the water in the lower layers of the Earth's atmosphere, circulating because of weather, washes the sulfur compounds out of these layers, whereas Venus has sulfur compounds in the lower layers of its atmosphere in addition to those in its clouds.