The predictable, regular rising and lowering of water in some areas due to the pull of the sun and the moon. Tidal changes can happen approximately every 6 or 12 hours depending on the region. To find out the time and water levels of different tides, you can use tide tables for your area. The period of high water level is known as high tide and the period of low water level is known as low tide. In the Bay of Fundy, the tidal range exceeds 40 feet (13 meters.)
There's several factors, the most noticable one is the gravitational effect of the moon, everything has gravity but the moon is very close to earth compared to the sun and other planets so it's effects are greter.
The moon rotates around the earth and together these, along with the other planets, rotate around the sun.
When the sun, the earth and the moon are together in a straight line is when the gravitational pull is at it's strongest - the moon takes about 29 days to go round the earth so this effect happens roughly each fortnight - once when the alignment is sun - moon - earth (new moon) and again when the alignment is sun - earth - moon (full moon).
The seas and oceans are so massive that the full gravitational effect of the moon takes a couple of days to peak and it's for this reason that the highest tides occur shortly after a full moon or new moon.
The sun also plays a part but to a lesser extent. The earth takes a year to go round the sun and the path it takes is an elipse - a slightly squashed circle, twice during the year we are closer to the sun and twice we are further away. The times we're nearest the sun are the autumnal and vernal equinoxes (the first days of spring and fall). Being closer to the sun means higher high-tides and lower low-tides and as a result the tides are at the highest and lowest at the start of spring and the start of fall.
When the full or new moon coincides with the vernal or autumnal equinoxes we get exceptionally high tides.
Other more localised factors are wind and air pressure. A strong driving wind will push the seas and oceans along in front of it (called a strom surge). An area of high pressure pushes down on the seas and oceans and displaces the water towards areas of low pressure - the low pressure areas effectively 'suck' the water upwards.
When several of these factors combine there can be catastrophic high tides leading to serious flooding and loss of lives. In 1953 a strom surge claimed over 2000 lives in England and the Netherlands and reclaimed over 2,000 square kilometres (800 square miles) of land.