Around 20,000 years ago as the last ice age relinquished its grip on the planet, enormous sheets of land ice began melting. Over several thousand years, the melt waters filled the oceans, raising sea level around 120 meters (394 feet) to near present-day levels. The rise leveled off about 3,000 years ago, and since the rate of sea level rise has hovered near 1 millimeter (.04 inches) per year.
In the 20th century, the rise began to accelerate. Today, the IPCC plots annual sea level increase at around 3 millimeters (0.12 inches) per year, likely due to thermal expansion of water and melting land ice as a result of global climate change.
Where will things go from here? Hard to say. One of the main problems with estimating future sea levels is that scientists don't entirely understand how quickly past sea level changes occurred, which would give them a model to work from.