The clean version of a message is shown by default and strips off all and previous replies that build up at the bottom of long email chains. The raw version is the entire message exactly how it was sent to the group.
If you don't want a message archive for your group, it can be completely disabled from your group Settings.
Messages are stored in the archive indefinitly. You can scroll down to see the most recent 1,000 and use the serach function to see older messages.
For groups on the free plan only the last three months worth of messages are stored.