There's no difference.
JAA is the Joint Aviation Authorities
JAR is the Joint Aviation Requirements
The first is the governing body - a sort of Europe-wide CAA; the second is the actual requirements that need to be satisfied to gain the license.
On the front of my license it says
"UK CAA, A full member of the Joint Aviation Authorities"
The license itself is a JAR-FCL PPL (A).
FCL is Flight Crew Licensing
A is Aeroplane (you probaly knew that)
Hopes this helps.