There's a lot of good information about tides at NOAA, the National Oceanic and Atmospheric Administration, which is a U.S. government agency. Here's the first page, about tides and the moon and sun:

The moon's gravitational force pulls the ocean toward the moon. The sun's gravitational force has only half as large a pull on the oceans as the moon's, because the sun is so very much farther from the earth.. So there is a tidal bulge, where the oceans bulge out towards the moon, on the side of earth closest to the moon.

Inertia has a big influence on tides, too. Inertia is the force that keeps things moving in the direction they were already moving. On the side of the earth away from the moon, where the moon's pull is smallest, the oceans bulge out because of inertia. There's a nice picture of the 2 ocean bulges on this NOAA page:

The moon, the earth, and the sun are all in a line when the moon is full or when the moon is new. Then the tides are higher and lower than usual, because the gravity, and the inertia, from the sun and the moon are lined up. These are called 'spring tides.' This happens every 2 weeks. One week later, at the half moons, the gravities from the sun and the moon partially cancel each other out, and the tides are not as high or as low. These are called 'neap tides'. NOAA has a nice animation showing this cycle:

You can check out the seasonal variations in the tides at this NOAA site:

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