current density is the flux of charge by an area. It's also the ration of the current and the area that it goes by.
Being more rigorous you must think about impressed, conduction and displacement density currents.
as tjunqueira said that current density flux of charge by an area
or in other words you can say that it is the amount of current passing through a
particular area..
Current density is a measure of the density of electrical current. It is defined as a vector whose magnitude is the electric current per cross-sectional area. In SI unit, the current density is measured in amperes per square meter.
Simply current intensity is the amount of charges passing a crossection in a second while current density for that curr. intensity is the amount of charges passing per unit area through that crossection in a second.
Being a vector quantity it can be related to the electric field as:
J = σ E
where σ is the material conductivity
To be right there are three types of currents :
1-conduction current (as that in conductors)
2-convection current (as in tubes)
3-displacement current(pseodo current in dielectrics)
for 1st type J = σ E
for 2nd type J = ρ v
where ρ is the charge density and v is the velocity
for 3rd type J=d D/d t
where D is the electric field density