|MadSci Network: Earth Sciences|
There are some really nice resources online to answer your questions about salinity. One place I often point students is Wikipedia, which has articles on both Salinity and Seawater.
As for your specific questions, here are some simple answers. More detail can be found in the above links.
1) Salinity is the saltiness or dissolved salt content of a body of water.
2) It contains mostly water, then, in order of decreasing importance, ions of chloride, sodium, magnesium, sulfate, calcium, potassium, etc.
3) The salts (i.e. the ions) come from rocks. Through leaching, erosion, out-gassing of volcanoes/hydrothermal vents, etc.
4) The salts increase the density of the water. Freshwater is typically 1000 kg/m^2 (it is temperature dependent) while seawater is typically around 1025 kg/m^2.
5) Volcanoes, along with hydrothermal vents, have contributed much of the chloride in the ocean.
I hope this helps.
Try the links in the MadSci Library for more information on Earth Sciences.