You may go for simple ones or more sophisticated systems, as per your needs. There are many types of watering systems, designed for both indoor and outdoor potted plants, as well as flower gardens. In short, such systems take care of the water requirements of plants and eliminates manual watering totally.
You can even make an automatic plant watering system from materials used for household purposes. Take a few plastic containers. Clean them well, and make two to three small holes on their bottom. Fill them with water and set them in the dirt, next to the plant.
If the soil is wet, very little amount of water will be absorbed from the containers; and as the soil gets dry, more water will be absorbed. Likewise, you can make such simple automatic plant watering devices using easily available materials.
They take care of the water requirements of individual plants. There are multiple plant watering systems, that can take care of a certain number of plants, at the same time. These systems constitute water tanks, pipes that supply water from the tank to individual plants, and drips attached to the end of each pipe.