Java Runtime Enviroment is something you can't live without now days.
in case you haven't already heard, Sun has released version 1.3, 1.4 and 1.5.0. after that they jumped to version 5.0.

in not rare case. applications still require for version 1.4. and new software are based on version JRE 1.5.
JRE 1.5 is not listed for download by default on Sun Java website. you may wondering where could find it. the truth is Sun JRE 1.5 absolutely equals Sun JRE 5.0. only the version number has changed. and you can still find it is version 1.5 after install JRE 5.0.
for example :
You can find Sun JRE 1.5.0_06-windows-i586-p-s.msi
by unpack the installation file of JRE 5.0 update 6.

same as above, Sun Java Runtime 6.0 equals JRE V 1.6.