Density

Density is anything that has both mass and volume.  The formula for it is D = m/v. 
Density is measured in many different ways.  Some common ways to measure it include g/cm^3, g/mL, an kg/L. 

Useful Info:     1 mL of water = 1cm^3 of water
                      density of water = 1000g/L
                      1 mL= 1cm^3= 1g/mL

Objects will float on liquids if they are less dense and sink if they are more dense. 
E.g. - during an oil spill, oil will float on the surface because it is less dense than the surrounding water. 

This is one of Bahama's blue holes.  They are underwater caves where dense salt water sinks and less dense fresh water(rainwater) moves above it.  The two layers do not mix.

Here's a cool video about the density of diet vs. normal pop. 
http://www.youtube.com/watch?v=MzsORE0ae10&feature=related

This clip shows sulphur hexafluoride, a gas 6 times denser than air
http://www.youtube.com/user/coolphysicsvideos#p/search/0/xQo-v_F1P9U