To put it simply, saltwater is water with salt in it. But the chemical composition is more complex.
Salts (both ordinary table salt and other salts) are chemicals that fall apart into electrically charged particles (called ions) in water. One big difference between salt water and plain water is that these ions make the saltwater conduct electricity much better than pure water.
In addition to water (made up of hydrogen and oxygen atoms — H2O), seawater in the ocean has more than 70 elements dissolved in it but only six make up more than 99% of all the dissolved salts and all occur as ions — that is, electrically charged atoms or groups of atoms: Sodium (Na+), Chloride (Cl−), Magnesium (Mg+), Potassium (K+), Sulfate (SO4−) and Calcium (Ca).
One of the really neat things about saltwater is that things float in it more easily than in regular water. For example, there's an especially high concentration of salt in the Dead Sea, so it's very easy to float there.