BSD sh vs. System V sh

[Keith Bloom]

I use a SysV clone (Venix) and BSD 4.2.  My login shell on both 
systems is Bourne shell.  Recently I took a shell script which
worked fine under Venix and moved it to BSD.  When I ran the script, 
I got the message "if: Expression syntax"

Based on a hunch (recollections of other scripts) I 
inserted a ":" at the top of the script (before the "if").
It worked.  I also tried inserting "#! /bin/sh", which also worked.

So the problem is solved, but I'd still like to understand why 
this was necessary.  Everything I've read about the : says simply
that it returns true - and so it does.  But apparently that's not
all it does.  Also, are there any differences between using : and
using #! /bin/sh  ?

(Before you tell me to RTFM: the BSD documentation I inherited is
 very disorganized, and parts of it are not there at all)

-- 
Keith Bloom        UUCP: {rutgers|ames|uunet}!mimsy!apclen!akbloom
                   Internet: akbloom at aplcen.apl.jhu.edu