what's the difference between innodb storage engine and innodb plugin

Can anybody tell me the difference between the innodb storage engine
and innodb plugin.it puzzle me for a long time!

The best resource for learning about this is on http://www.innodb.com/ where there is a clear description of the differences and advantages.

How can i know mysql’s default innodb version!For example,if i have a mysql versioned 5.1.37,how can i know it’s default innodb version!