What is the primary cause of tidal activity on Earth?

The primary cause of tidal activity on Earth is the relative relationships between our planet, the moon, and the sun. Both the sun and the moon have gravitational pulls that affect the liquid water on Earth. Although the sun is much larger than the moon, and therefore has a greater gravitational pull, it's also much farther away than the moon. The relationship between the sun and Earth is pretty steady, so the sun's part in tidal activity is actually quite minor. Much of the variation we see in tides is the result of the moon's orbit around our planet. In many cases, the gravitational pull of the sun and moon conflict with each other; during an eclipse, however, the pull of the moon and sun combine to create the biggest tides experienced on Earth. 

Tides vary regularly throughout the day as a result of the Moon's orbit around our planet. Over longer periods of time, the Earth's rotation around the Sun affects the relative intensity of tides.