Single Instance of Backend

I have heard several mentions of the requirements for single postmaster. Is a postmaster 'the backend'? Maybe I could define what I think is the current situation first:

client1 client2 client3 ....
| | | |
+-----------+-----------+---------+
|
V
Postmaster
|
V
OperSys
|
V
Disks

Is this correct? Is this because of MVCC? How many other DB's are using MVCC?

To make clustering work, and be as redundant as possible, how do the clustering solutions work for:

Postgres?

Oracle?

DB2?

Other Real DB's?