There are two tides ( in most places anyway ) a day because the Moon does not orbit the Earth, the Earth and Moon orbit a common centre of gravity, which is inside the earth, but in a direct line between the moon and the centre of the earth.

The tide nearest the moon is due to the gravity of the moon pulling water towards it. The one on the opposite side of the earth is due to the body of the Earth itsaelf being pulled toward the moon more strongly than the water on the opposite side of it.