Why are there tides?  The answer is not obvious unless your observation of nature has led you to notice a relationship between the moon and high tides.  The solution lies in the gravitational interaction between the moon, the sun, and the earth, along with rotation of the earth, and the orbits of those bodies. That combination results in t
wo daily bulges in the ocean level that moves around the earth following the moon.  One bulge is opposite the moon (due to the pull of gravity) and the other on the opposite side of the earth (due to centrifugal force of earth/moon system rotation).  When the bulge arrives in Puget Sound it causes a high tide and when it leaves, the tide is low resulting in two series of tide changes per day.  The height of the bulge varies daily depending on the relationship of the sun to moon. When the sun and the moon are aligned with the earth the gravitational pull is greater and the high and low tides have a much greater range (called a "spring " tide, meaning "to jump").  When they are at 45 degree angles then the pull is less and the tide range is less, the so called neap tides.  The change occurs about every seven days.  The tidal range also varies with local bathymetry.. 

The tide can be predicted using mathematic equations which are generally reliable but can differ from reality due to weather conditions: for example a storm can result in a higher tide than predicted due to the low atmospheric pressure. 
 Let's use the Boston Harbor tide chart to discover what the expected tide level will be.  The two mountains represent high tides with the valleys representing low tides.  The chart axis on the left is height of the water level.  The bottom axis is time from midnight to midnight.  The first high tide is predicted to be at around 4:29 am with a height of 11.69 feet above mean lower low water (the 0.0 tide line or tidal datum).  Then 6 hours and 15 minutes later will be the low tide of 2.02 feet at 10:43 am and so on through out the day.  The extra 15 minutes is due the lag in the moon's orbit relative to the earth's rotation making the next pass of the moon a total of 50 minutes later each day.  If a low tide is below the 0.0 tidal datum it is referred to as a minus tide.

Current local tide predictions can be found here.

What does this mean to the animals and plants of the intertidal zone? 

The tide chart indicates that over this 24 hour period an organism like a speckled limpet or rockweed living at the 11.00 foot tide level would have been dry for 17.25 hours out of 24 hours or 72% of the time.  An organism like a clam or shore crab living at the 3.00 level would have been dry only 14% of the time. 

Most of the low gradient beach, as shown on the below photograph, is covered with water more than 50 % of the time (below the mean sea level mark of a +8.2 ft).  The extreme low tide line at a - 4.5 ft marks the boundary between the intertidal zone and the subtidal zone.   Note that some of the boats are going aground as the water recedes.  The low water line in the photo is around the - 3.0 ft mark.  The green seaweed, Sea Lettuce, covers the beach from around the +5.0 ft line to extreme low tide.  The Turkish Washcloth seaweed and Rockweed grow above this line.  There are fewer clams and Mud-flat crabs above this line as well.  (Note: the indicated tide levels are based on mathematical estimates rather than measurements)