A mirrorless camera is exactly that...its missing the main mirror. My appologies if the following is too basic.
What makes an SLR an SLR is that the eyepiece you look through is sharing with the capturing media, be it film or sensor. There are actually two ways to do this, one would be to divide the light with a prism, but that not very effective and reduces the light you see and the light the sensor sees. The second option is to use a mirror. When its in a down state, the light coming through the lens get reflected up to the view finder. When you press the shutter button, the mirror flips up and the light get sent towards the film/sensor. (I'll ignore the shutter curtain...if you don't understand that you'll need to look/ask seperately).
On mirrorless cameras, the light always hits a sensor. Then you look at a monitor. As someone said on another post, if you are using your live-view option on the camera, you are effectively using a mirrorless camera. Mirrorless cameras could also offer a conventional looking viewfinder, but you would actually be looking at a tiny LCD.
Hope it helps.
There isn't anything inheriently better/worse about a mirrored/mirrrorless design, however most screens can't currently show you the detail that looking through the lens can. There is also a slight delay in the sensor reading the image and the screen showing it. Its nearly imperceptible, but its there. There are several advantages. Mirrorless is mechanicly more simple and it doens't introduce some vibrations. You would think that the displays would add cost, but the simplicity of not implementing the mirror and its optics can be a cost saver. Time will tell.
Thanks. thats actually what i thought it meant, but i figured it had to be more since so many companies are trying to make mirrorless cameras. I'm not sure what the benefit(s) is(are)? Less camera shake when you fire the shot? Smaller?
Right now i have a 50D, and even when taking macros with the 100mm (non L) I don't have any problems. If i'm really picky I go to live view on a tripod with a 2 second delay and a remote trigger!!!
I hope that doesn't mean we will have to look at everything through the LCD screen - I like my viewfinder!