Tides are mainly caused by the effect of the moon's gravity on the Earth's oceans. Bodies of water on Earth that are closer to the moon experience a slightly stronger attraction to the moon than the far side. This causes a tidal bulge on the side that's nearer to the moon, and thus high tides on that side. On the far side, we get a tidal bulge and thus high tides for a different reason! Even though the waters on the far side of the Earth are attracted slightly less, the Earth itself is solid and is slightly more attracted to the moon. This difference in attraction to the moon leads to a relative acceleration of the waters on the far side away from the solid crust of the Earth, which means it "bulges" slightly on the far side, creating high tides.