Formerly mass and volume were linked by defining the gram to be the mass of one cubic centimetre of water which meant that water had density 1 kg/litre. Now length (that is the metre), and hence volume, and mass (that is the kilogram), are defined independently of water so the density of water is not quite exactly 1 but it is still very close. A cubic metre of water thus weighs a ton.
1000 kg m-3 is 1 g cm-3.
The perhaps highest density known is reached in neutron star matter. A black hole, according to current theories, does not have any volume, and its density can be seen as either infinite or non-existent.
The densest naturally occurring substance on Earth is osmium, its density is about 22 kg/litre.
This use of density is also known (if you follow [ISO 31]?) as volumic mass (that is, mass divided by volume).
Density may denote how much of a certain substance, object or occurrance is present within a certain area or volume. Often used is population density, meaning how many people on average live in an area (usually persons per square kilometre).