Significance of metastore_db

[Josh Ferguson]

One more thing, can someone explain to me what metastore_db is and  
what its significance is, apparently if I run hive from separate  
places certain instances can find some tables and some can't. I'm  
assuming this metastore_db is supposed to be shared around, and this  
it is updated at some point and maintained somehow? Can someone  
explain this piece of the architecture to me?

Josh