up to my knowledge
jre ---
java runtime environment means it is the java platform which helps to run the programs written using java on your platform(computer)
jdk --- java developer kit means it is the developmet kit used to develop java programs
j2sdk --- means java2 standard development kit.this is standard development kit .we have enterprise edition(enterprise apps) and micro edition(mibile apps).
finally from 1.2-1.4 version java was cosidered as java2 and now it is java6.0(1.6.0_x)--(
SCJP 6.0)