Tides are caused by the gravitational forces of the Moon and, to a lesser extent, the Sun pulling on the world's oceans. These gravitational effects cause a bulge of water on either side of the Earth (gravitational attraction on the 'near' side and centrifugal/centripetal force on the 'far' side) . As the Earth rotates we pass into and out of these bulges of water; the sea near us rises and falls giving us tides.
How big the bulges are and where they are depends on the position of the Sun and the Moon relative to the Earth. Watch how the bulges of water around the Earth change in size and position as the Moon orbits the Earth.
Spring and neap tides
When the Earth, Sun and Moon are in line with each other, the Sun and the Moon pull together on the same part of the ocean, so the bulges of water are bigger. This means as we pass through the bulges of water the sea rises higher and when we are out of the bulge the sea also drops lower as a lot of water has rushed to be in the bigger bulges. These are called spring tides (nothing to do with season of spring). The effect is strongest when the Earth, Sun and the Moon are directly in line.
When the Sun, Earth and the Moon are not in line, the Sun and the Moon are pulling against each other on different parts of the ocean, so the bulges aren't as big. So, as we pass through the bulges the sea near us does not rise or fall as much; the tidal range is smaller. The tidal range is smallest when the Moon, Earth and Sun form a right angle. These are called neap tides.
Flood and Ebb
A Flood tide is when the sea level is rising, an Ebb tide is when the sea level is falling.