Excerpt from Rakhesh Sasidharan <rakhesh@cse.iitd.ernet.in>:

First for a bit of background: When you have a disk, you generally divide
it into "primary partitions" and "extended partitons", and then furthur
divide the "extended partitions" into "logical partitons".  An extended
partition is just like a container for all its logical partitons.  This is
the way the Windows world works, and also Linux.

The total number of primary+extended partitions must be FOUR, and you can
have at most ONE primary partition.  And depending on the OS, you have
around 15-16 logical partitions (I'm not quite sure of this, but I'll get
to this later on).
(snip)

One hard disk can have more than one primary partition!  I think actually the
total number of primary + extended partitions must be <= 4, can be as low as
one, which would be the case where Windows is installed to one partition
spanning the entire hard disk.  I suppose one could have a NetBSD slice spanning
the entire hard disk?  As far as I know, extended partitions are limited to at
most one on one hard disk, but there can be up to four primary partitions with
no extended partition, or three primary partitions with one extended partition.
Would a (Free, Net, Open)BSD slice correspond to a primary partition as far as
the main (BIOS?) partition table is concerned?  I suppose one could conceivably
have four primary partitions and install DOS, FreeBSD, NetBSD and OpenBSD, with
some boot selector in the MBR?