You hear it all the time on television, the radio, and in various print media. The sea levels are rising due to a man made phenomenon known as global warming. Unfortunately, the link between these two concepts is often misunderstood, if it is presented at all.
The first concept the must be illustrated is the infamous green house effect. This effect dictates that there is a layer of gas around our Earth, and it keeps certain things in and certain things out. One of the things it is designed to keep in is the rays of the sun, or at least some of them. This has the effect of giving our earth a relatively constant temperature, which is what makes our planet habitable in the first place.
Unfortunately, due to the increase in fossil fuel burning, more gas is being ejecting into our atmosphere. This has the unwanted effect of keeping more sun rays inside the atmosphere than we would otherwise like, because the amount of rays coming from the sun is staying the same. Thus, if we conclude that the Earth is receiving the same amount of sunlight, and more sunlight is remaining trapped inside our atmosphere, the Earth will get warmer in proportion to the amount of sunlight it receives.
It should also be known that at each North and South pole exist large masses of ice. These are normally frozen rock solid due to the extreme polar temperatures. However, with increased heat from the aforementioned greenhouse effect, the only logical step to take is that these "ice caps" as they are so called will start to melt.
To make the final connection between the melting of the polar ice caps and the rising sea levels, there is a simple experiment one can perform. Take a plastic glass of water, and place an ice cube in it. After sufficient time has elapsed, you'll notice that the water level in the glass has risen. This is because the molecular bonds inside water molecules have been broken, forcing it into the liquid state, and it goes into the ocean. When you have millions of tonnes of ice melting like this, the ocean levels will undoubtedly rise.
Many people will contend that an equal amount of water will evaporate, thus negating the change in sea level. This is simply not true. Liquid water has what is called a high specific heat capacity, which means an awful lot of heat must be absorbed before it will shift into a gaseous state. Thus, the water levels will continue to rise, until the ice caps meet the same fate as the ice cube in the glass of water.