Earth depends on both the Sun and the moon for life as humans know it to exist. The Sun is the ultimate source of energy for the planet, and the moon provides a slight break against the Earth's rotation via tidal forces. The Sun also affects tides.
The Sun provides heat and light to Earth. The heating of Earth's surface produces wind and warms the soil and oceans. Warm air and cooler air intermingle and create rainstorms and other precipitation, and the light gives plants the energy they need to conduct photosynthesis and play a vital role in the food web. Because of the Sun's gravitational pull on Earth, it produces tides, but not as intensely as the moon does.
The moon's gravity and the Sun's gravity align in the new and full moon phases, creating larger tides that slow Earth's rotation slightly. The tides carry water around the oceans and fuel currents that transfer heat from the equator toward the poles. The gravity of the moon is also thought to keep the Earth from tilting too far and therefore making its climate and seasons more stable. If the axis of Earth were straighter, the seasons would be less noticeable. If it were tilted more, the seasons would be more intense.