Definition
Tides are the regular rise and fall of sea level. They happen because of the difference between the gravity of the Moon and Sun pulling on Earth and the centrifugal force from the Earth-Moon orbital motion. In many places, including the seas around Korea, the sea rises and falls about twice a day, but in some places, depending on the shape of the sea, it happens only once a day. The flow of water moving in toward land is called the flood tide, and the moment the sea reaches its highest level is called high tide. The flow of water moving out is called the ebb tide, and the moment the sea reaches its lowest level is called low tide.
The Moon pulls on the ocean
Spread out a beach mat on the sand and play for a while, and you may find that the waves have crept up to your feet. The water isn't sloshing around on its own. Huge objects floating in space are pulling on Earth's oceans. The closest of them, the Moon's gravity, plays the most important part.
The ocean on the side facing the Moon feels the Moon's gravity strongly and bulges up toward it. The ocean on the opposite side of Earth is farthest from the Moon, so the pull is weaker there. But the centrifugal force from Earth and the Moon circling together pushes that water out in the opposite direction, so it bulges too. In the end, Earth's ocean water forms a rugby-ball-shaped oval stretched out on both sides.
Earth spins once a day, so we pass through these two bulges of ocean about twice a day. The flow of water moving in toward land is the flood tide, and the moment the sea reaches its highest level is high tide. The flow of water moving out is the ebb tide, and the moment the sea reaches its lowest level is low tide. Keep in mind that one set of words names the flow and the other names the moment.
The Sun joins the tug-of-war
The Moon isn't the only one in this cosmic tug-of-war. The Sun is much farther from Earth than the Moon, but it is so enormously big that it pulls on the water too. Still, the force that creates tides (the tide-generating force) weakens very quickly as distance grows. So the Sun's tidal force is only about 45% of the Moon's, which is much closer.
There are days when the Moon, Earth, and Sun line up in a straight line in space. That happens around the new moon and the full moon of the lunar calendar. Then the pull of the Moon and the Sun add up in the same direction and tug the water as hard as they can. The result is the spring tide, when the gap between high and low tide is the biggest of the month.
At other times, such as the first-quarter and last-quarter Moon, the Moon and Sun make a right angle with Earth at the center. Then the two bodies pull the water in different directions, and their force is split up. The pulls partly cancel out, and the neap tide arrives, when the gap between high and low water is the smallest.
To be a little more precise
It's easy to think the Moon's gravity alone moves the water. More exactly, it's the tide-generating force, which combines the difference in gravity felt at each place on Earth with the centrifugal force of the orbit. It acts on more than water. Earth's solid rock crust feels this force too, and it rises and falls slightly, by tens of centimeters a day.
Tides also make a big difference to sea life and to our daily lives. The wide tidal flats that are exposed at low tide are home to countless marine creatures, and they work like a septic tank that filters and cleans the sea. On Korea's west coast, where the difference in sea level is large, tidal power plants use this force to generate electricity.
The friction made when seawater hits the land is also slowing Earth's spin ever so slightly. Because of this, the length of a day is slowly getting longer by about 0.002 seconds every 100 years. That's another surprising result of the tides.
Common misconceptions
Tides happen only because of the Moon's gravity.
The Sun's influence is also big, about 45% of the Moon's. The difference between high and low tide changes in a regular cycle depending on where the Moon and Sun are relative to each other.
Where you meet it
Someone digging for clams on a tidal flat on Korea's west coast has to head out before the flood tide arrives and the water rises.
This is how Korea's Sihwa Lake Tidal Power Station works: it traps the big difference in water level between flood and ebb tide and uses it to spin turbines.
In one sentence
Tides are the regular rise and fall of sea level (about twice a day in many places), caused by the Moon and Sun pulling on the ocean.