There is nothing simple about understanding tides, because so many local factors - latitude, wind, currents, contour of the shore - come into play in addition to the sun and moon. Up here in New England, you generally need to check a tide table before planning any water-related activities - swimming, boating, clamming, and fishing.
For example, on Cape Cod spring tides can entail an 8-10' tidal change, leaving boats sitting on mud flats at low tide. On the ocean beaches, you want to frolic at low tide, not at high tide when the currents and undertow are strong and you cannot find a footing (I didn't mention all of the Harbor Seals on the ocean beaches this year. Cool. You can paddle right around those big fellas. I think that they think that we are a different seal species.). On the harbor and bay beaches, you cannot walk far enough out to find water deep enough to swim in at low water, although the dogs love to splash in those shallow mudflats.
As you move towards the equator, tidal changes become less important. Do you know why? (Here's why)
Spring tides are the extreme variations in tidal change. Neap tides are the minimal variations. They each occur roughly every two weeks. It's all about how the moon and sun line up to pull the water in the semidiurnal tidal cycle. Wiki has a good summary.
Here's my photo of Wellfleet oystermen tending their precious oyster plots during a spring tide low water last week in Wellfleet. They take their pick-ups out onto the mud flats, and hope they don't get stuck as the tide comes in. At half-tide, you can swim and sail there.
No boat's gonna row: Low tide in the Wellfleet marina:
post to del.icio.us