The water doesn't have to be an elaborate waterfall or a themed fountain. Just a simple dish on the ground will suffice, and will please those birds that favor staying on the ground. If you want, a second dish at a raised level will be more comfortable for those birds that don't like going down to ground level.
Water in the summer is key for keeping birds healthy and not dehydrated. The water helps keep them cool and keeps off the dust and dirt of the world. In the winter, water is key, since most natural water is frozen into ice or snow. Providing liquid water may be the difference between life and death for some birds.
Make sure the water isn't deep enough to drown in. It should be just a few fingers high, and you might want to put a flat rock or two in it to give birds a place to land on that's dry. Change the water when it gets dirty. Birds, just like humans, don't enjoy drinking mud or slime.
Birds do love the sound of running water, so if you can hook up any sort of system that involves moving water, you'll thrill them! One simple thing to do is to place your water dish underneath a tree. Take a plastic soda bottle, fill it and cap it. Poke a small hole in it, just enough to let drips out. Now hang the bottle over the dish from one of the tree branches. The drip-drip-drip from the soda bottle into the dish will be a siren call to the birds, letting them know that water is nearby.
How to Choose a Birdbath
Attracting Birds with Food Choices
Bushes and Shrubs that Birds Love