There are several ways that plants can use to conserve water. First the epidermis of the plant especially the upper epidermis is covered by a waxy cuticle. The cuticle is quite obvious in this shiny oak leaf. 
 
 

In addition, many plants especially those we call succulents have thick leaves that are often reduced in size. These plants have extra ground tissue for water storage, and the reduced leaf size means less surface area from which water can be lost. A good example of a succulent is this jade plant commonly grown as a house plant or outside in warmer parts of the country such as in Southern California. 

 
This plant from the South West is called the pencil plant because of its round green stems. The stems carry out photosynthesis and the leaves are very reduced in size. Again this is an adaptation for water conservation while allowing at least some photosynthesis to take place. 

The plants we call cacti carry the process of leaf reduction one step farther. When we look at a cactus such as this Opuntia, commonly known as "Prickly Pear", the green fleshy pads are actually modified stems with lots of ground tissue for storage of water. Where are the leaves? The leaves in all but one species of cactus have been modified through evolution into structures that we call thorns! 

Thus we see, that the amount of water available to the plants is a critical factor shaping the evolution of plants in different environments. Indeed, what biologists see when they look at plants is that unrelated plants in different parts of the world but under similar conditions of water abundance and temperature have evolved similar forms. Thus to a casual observer the succulents in South Africa will look superficially similar to those say in the deserts of North or South America, even though the plants are unrelated! This similarity of form related to similarity of environment for unrelated species is called convergent evolution.