The root zone of every plant requires three things: air, earth and most of all, water. Water is essential for the plant's uptake of soil nutrients. Moisture in the soil also governs how deeply a plant sends its roots into the earth, and this in turn is related to its ability to withstand drought. There is more to watering than simply turning on sprinklers. Dwindling resources demand you water efficiently, too.
Over-do it and the plant will drown; under water and it will wilt. To be sure, don't just look at the soil, dig a little hole and find out exactly what's going on down under. If you find black soil and roots that smell badly, you have watered too much. This may also indicate you have poor drainage, and water is just not percolating down and out of the root zone. If it's dry down there, then increase watering or change your strategy. Water more frequently, or water for a longer duration.
The best way to know how water behaves in your soil is to dig a test hole one to two feet deep and fill with water. The speed in which it percolates down into the soil is a gauge of its drainage.
The best way to water is to apply it at a rate that the soil will absorb without runoff. It must be able to saturate the soil many feet below the surface. This is especially important with new plants. By watering deeply the first season, you'll help your plants grow deep root systems, able to access moisture far underground after surface soil has dried out. This quality can also be encouraged in more thirsty plants to make them healthy and as water thrifty as possible. The result is you water less often, and the water you do apply is used much more efficiently.
Mulching is the best way to conserve water. Mulch is simply organic material such as ground bark or wood chips that's spread out on top of the soil in a layer at least 2" deep. Mulches insulate roots, discourage weeds and eliminate surface moisture evaporation from the soil.