There are lots of different types of camera out there, from pinholes that you can make yourself using a cardboard box, to large format cameras that use one sheet of film per exposure, via instamatics and 35mm single lens reflex (SLR) cameras. Recently, however, there has been a trend towards 'mirror-less cameras'. They are commonly seen as a bridge between compact cameras and SLRs, but there's a bit more to them than that.
What's in a name?
First, we need to start with the term 'mirror-less camera', because it isn't entirely accurate. You see, a compact camera is mirror-less, but because these new-fangled beasts are kind-of modelled on SLR technology, the lack of mirror becomes important. You might also hear them referred to as DILs (Digital Interchangeable Lens cameras); EVIL cameras (Electronic Viewfinder, Interchangeable Lens); Micro cameras; MILCs (Mirror-less Interchangeable Lens Cameras); PEN cameras (that's the name Olympus gave them); or SLDs (Single Lens Direct view cameras).
Yes, I agree that it is very irritating and confusing when the manufacturers can't settle on a term. As much as I love EVIL, we'll stick with mirror-less. And you might be getting the picture from the names that mirror-less cameras have interchangeable lenses, just as SLRs do, but no mirror.
Understanding how a mirror-less camera works, and why it is regarded as a significant step forward in camera manufacture, is pretty simple. But first you need to understand how an SLR camera works. Thankfully, that's pretty simple, too.
When you look through the view-finder of an SLR camera, you're seeing exactly what your lens is seeing, at the speed of light, via a mirror and a pentaprism. The mirror reflects the image that is coming through the lens up to the pentaprism, the pentaprism flips the image the right way around (otherwise it'd be up-side down), and then you see it through your optical view-finder. When you press the shutter release button, the mirror flips out of the way and exposes the sensor (or film, if you're that way inclined) to the image. There's your picture.
A mirror-less camera does away with the optical view-finder and instead has just an electronic view-finder. (You know, the LCD screen on the back of your camera.) So instead of seeing a reflected image, you're seeing an electronic image. That means the camera doesn't need the added bulk of the mirror and the pentaprism, either. Thus, you've reduced the size and weight of the camera, but you still have all the fun of playing with interchangeable lenses, which you don't get with compact cameras.
Lots of lovely lenses
Even better, you get to play with all kinds of lenses with a mirror-less camera, not just the ones that have been made for your particular make of camera. You see, a Canon lens won't fit on a Nikon body without an adapter. (An adapter is a ring that allows the lens to lock into the body.) And when you use an SLR, an adapter puts the lens too far away from the sensor, so it messes up the focal point. But the smaller size of the mirror-less camera means that you can use an adapter with impunity. Lots and lots of lovely glass is now going to fit on your camera.
Does this seem too good to be true to you?
Well, sort of. If you're stepping up from a compact camera and are accustomed to using only an electronic view-finder (that's the LCD screen on the back of your camera), but are looking forward to playing around with wide-angle, and telephoto, and macro, and fisheye lenses, you'll probably not notice much difference.
If you're like me, though, and have been shooting on an SLR for long time, you'll be accustomed to seeing exactly what your lens sees at exactly the same time that it does. If you're a sports photographer, that delay between the moment of action and seeing it on the electronic view-finder will be the difference between a picture that's in all the sports sections of the Sunday papers, or one that's sent to the delete bin.
With a mirror-less camera you lose the bulk of the SLR, get lots of lens flexibility, but you also lose its speed. By the time that you've added a lens, it's substantially bigger than a compact camera, but of course you have the lenses. So really, it's a bit of give and take.
So what do I think? I'm biased. I learned to use an SLR when I was seven years old. I won't be giving up mine any time soon. If you're not confident enough to make the leap to a dSLR (and the Nikon D3100 is excellent value for money) stick with a high-end compact. You can't go wrong with a Canon S95.
All of the research for this was done when I wrote 'Where'd my mirror go?' for Photocritic. It's a more ranty, more technical version of this.