How do lightguns work, and how is the Guncon different?
Charles Doane posted this excellent description to the group:

The simplest answer is that the TV has a scan, called a "raster" that
moves from the top to the bottom of the TV screen as it sweeps from left
to right. It does this 60 times per second (USA/Japan), the standard is
called NTSC (National Television Standards Committee, or Never The Same
Colors).
The light gun can see that scan, and by comparing when the scan goes by
against one of the synchronizing signals, the software can determine the
position of the gun.
The reason you can't see that scan is a trait of the human eye known as
"persistence of vision", which is why flashbulbs tend to make spots in
your vision for a little bit.
The lightgun doesn't have that limitation, it's a machine. Cameras don't
have that limitation either, which is why, in some photos with a TV on,
it appears that the TV screen is partly blank. The reason it appears that
way is because the TV screen really is partly blank much of the time, you
just can't see it because of the way your eyes work.
The light gun is just a light receiver, it can no more hurt your TV than
your eyeballs can, so don't worry about that at all.
As for the best brands, it depends. For Namco games, you're stuck with
either a Namco Guncon or a Clone such as Nyko's Super Cobra, or for the
earlier games (like Die Hard Trilogy or Project:Horned Owl) you need a
non-Guncon type of gun (The Nyko Super Cobra works for those too). There
were two gun standards for the PSX in one of the few screw-ups by SONY,
but now it seems that the standard has pretty much centered on the Guncon
for all recent and future PSX gun games.
P.S. The reason the Guncon has the Composite Video plug is that it uses
the far more frequent (and therefore more accurate) Horizontal sync,
which occurs 262.5 times more often than the Vertical sync's 60 times per
second, or 15,750 times per second. Pretty spiffy, huh?