i want to get one of the new fangled red dot sights.

How does one use it and in what situations?

Thanks

Jim

When you look though the tube, there is a red dot image projected on a piece of semi-reflective glass. The red dot is generated by a lazer diode.

How to use it?
Put the red dot where you want the bullet to go.

The magnification is typically unity-gain (1x) so they are best used for relatively shorter distances. Advantages are; low cost, gobs of eye relief, and quick target aquisition.

I'm no expert, but I just bought one myself not too long ago.  It's mainly useful for short to medium range work where quick acquisition is important.  Home defense, police work, and hunting, for example.

They generally don't magnify, acting much like iron sights.  The advantage over iron sights is that instead of having to focus on three different things at three different distances (rear sight, front sight, target), you now have everything on one sight plane.  Just look at the target.  The red dot will be in your vision and pretty much in focus.  Just put the dot over your target and squeeze the trigger.  the advantage over standard scopes is that you don't have to worry about eye relief (i.e. your eye doesn't have to be a certain distance from the rear of the scope).  You can look through it from 1 inch back or 2 feet back.

The better red-dot sights have specially made lenses that keep the dot correctly lined up with the target no matter how you're looking through the scope.  As long as you can see the dot, the bullet will hit what's behind it.

I haven't had mine (Aimpoint Comp-ML2) long, but I can say that so far, I love it.

They help with fast target aquisition, because you can keep both eyes open.  

Both eyes open, long eye relief and low to no magnification means that you can really get on it fast.

I have recently purchased the Eotech Holosight.  Instead of just a dot, you can get many different recticles to aim with, like dot inside a circle, which I use.  Like has been said before, There is no magnification, so useful in short and medium range situations.  Two eyes open is the serious advantage.  And the Holosight is parallax free, so I have a backup flipup rear sight in the event of a sight failure, I can go to iron sights in a second.  Excellent setup.  Good luck in your purchase!