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Both point-and-shoot camera and mirrorless cameras seems to be mostly similar in the way they work.
So, What are the differences between a point and shoot and a mirrorless camera?
Is a mirrorless camera a point-and-shoot with big sensor, an electronic viewfinder and interchangable lens? If so, why call it a whole new product variety?
Technically mirrorless means there is no reflex mirror for a through the lens optical viewfinder and instead an image is read from the sensor in realtime and displayed on some form of LCD.
However since this applies to all P&S cameras the term is usually used to refer to mirrorless cameras that feature interchangeable lenses, such as micro 43rds, the Sony NEX range, the Nikon 1 series, Pentax Q and recently Canon's EOS M.
Another feature common to mirrorless cameras is a larger sensor, though this is not a defining feature as there are exceptions, such as the Pentax Q, a small sensor mirrorless, and various larger sensor P&S cameras, e.g. Sigma DP2, Canon G1X, Sony RX100.
Other terms used to refer to the same group of cameras include:
Though none of these terms are defined rigorously, for example it could be argued the EOS M is not an EVIL camera as there is no electronic viewfinder per se, only a rear LCD screen.
I think part of the name confusion stems from the fact that today's "mirrorless" cameras are a fairly recent introduction in digital photography. Where we once were able to distinguish point-and-shoot cameras from DSLR cameras pretty easily, the trend in the industry has been to blur those line more and more.
So-called "bridge" cameras might be seen as the first salvo here, offering improved optics and features (compared to traditional P&S cameras) without making the jump to multiple lenses and (in most cases) large sensors.
The term "mirrorless" is most frequently applied today to Mirrorless Interchangeable Lens Camera (MILC) systems or Electronic Viewfinder Interchangeable Lens (EVIL) systems. In these cases, the two distinguishing characteristics are the lack of a moving DSLR-style mirror and the ability to change lenses. Sony's translucent mirror technology might be considered a fringe member of this class, though I believe its feature set and target audience, plus the fact that it does, in fact, have a mirror, probably place it closer to traditional DSLRs.
A final consideration for these cameras is the sensor. Although mechanically, these cameras resemble P&S cameras, many of them feature sensors more closely resembling (or identical to) crop-sensor DSLRs, vs. the typically smaller sensors found in P&S cameras. Again, you can't draw perfectly clear lines here, since there are some P&S cameras with larger sensors (Canon's G-series comes to mind). In some ways, in fact, these could be seen as predecessors or inspiration for some of today's mirrorless offerings.
