(it's trickier than you may think!)

Over-watering kills plants.
Under-watering kills plants.
Research each plant's requirements.

Generally the soil should be moist.
Not too wet. Not too dry.

The soil should be like MOIST CAKE
Soggy cake kills plants. Dry cake kills plants.

Roots need moisture and they need air!

Use Caution when watering newly transplanted seedlings!
It is easy to drown them! Give them time to grow roots into the pot!

Plants that have an established root system require more water.
Hot, sunny, and windy weather means your plants require more water.

Do NOT water on Cold and cloudy days unless very dry!

Watering in the morning is preferable to allow time to dry before nightfall.
Wet plants at night can become diseased, moldy, and/or rot.

Water just enough to saturate the soil - so just a little water runs out the drainage holes.