What units are used in ABAQUS?

There is no inherent set of units in ABAQUS. It is up to the user to decide on a consistent set of units and use that units. Typical sets of units : 

                    1            2

Length          - metres        mm
Force           - Newtons      Newtons
Time            - second       second

Mass            - Kg           tonne (**)  
Density         - Kg/m3        tonne/mm3
Stress          - N/m2         N/mm2 (= MPa)
Young's Modulus - N/m2         N/mm2 (= MPa)

** 1 tonne = 1000 kilograms

Decide on the units before you start preparing your data. This is critical. If you start typing in the nodal co-ordinates that means you have already decided on what units to user for the Length parameter. This only leaves the choice for the units of Force. 
It is not a good idea to choose mm for length, Newton for force and then specify the Young's modulus in KN/m2.