The tides of large bodies of water and the ocean are caused mostly due to gravitational pull of the moon and partially due to gravitational pull from the sun. As the Earth rotates and spins on its axis it gets closer to the sun during the day and closer to the moon at night. The sun is very, very far away but can still exert some gravitational influence. The moon is relatively close and has a large influence with respect to gravitational pull.
Each day there are high and low tides because of the imbalance of gravitational pull. When the Earth is closer to the moon the oceans are pulled toward the moon. This pull can create high tides in some places and low tides in others. The places where the water is pushed inland are experiencing a high tide while the places where water is pulled away experience low tide. These tides can be drastically different. A channel can be nearly empty at low tide and filled to the top of the dock at high tide.
Inertia also plays a role in the tides. Inertia is the resistance to any physical object to change its state of motion or resting state. This means that the gravitational pull has to overcome the force of inertia in order to cause a tide. The moon has a rather strong gravitational pull and it therefore can easily overcome the force of inertia of the water. The sun does have some influence but has a harder time causing tides because of its distance.
The resultant force of gravitational pull can cause bulges of water. There is one when the moon and Earth are closest to each other and another when they are the farthest apart. As the season change, so does the angle at which the Earth and the moon near each other. This is known as declination. During the summer and fall equinoxes, the moon has the least influence on the tides because it is closest to the equator. During the summer and winter equinoxes, there is more influence from the moon.
The lunar day is twenty four hours and fifty minutes, slightly longer than a solar day as it is measured on Earth. As a result, some regions can experience to tidal bulges each lunar day. This is why there is a high tide every twelve hours and twenty five minutes. This is also why tidal charts are necessary. The charting system must take into account the season, the closeness of the Earth to the moon and the cycle of the lunar day.