1) What is known as "EMSMDB" ?
EMSMDB refers to the Exchange transport provider, EMSMDB32.
One example of a provider is EMSMDB2, the "Electronic Messaging System Microsoft Data Base, 32 bit build" provider. EMSMDB implements both a transport and a message store and, as such, is a dual provider.
The transport is the ability to submit messages to Exchange Server; the message store is the ability to read (and possible write) messages to an Exchange store process.