An electronic viewfinder is a device you will find on some digital still cameras, and on most camcorders. They are often abbreviated to EVF or EV-F
On digital still cameras, they are most often found in cases where a manufacturer has tried to make an SLR camera, but failed along the process: on an SLR (Single Lens Reflex) camera, the light goes through the lens (single lens), via a few mirrors (reflex...) to the viewfinder. This is often seen as the best way to take pictures, as you see exactly what the picture will be like, and the picture moves in real-time (well, at the speed of light, at least). Many viewfinders have only 98 % coverage, but still: Generally speaking, you will see what the focus and framing will be like. Also, you don’t have a parallax problem.
Due to the construction of many digital cameras – especially on long-zoom cameras, the “true” SLR design could not be used. Also, using a regular rangefinder viewfinder when you have a 40-400 mm zoom lens (which is the case on the Canon Pro90 IS digital camera), is a complete mistake. This means that you would have to rely on the little LCD monitor on the back of the camera. Fine and well, but if you have ever tried to look at one of these in bright sunlight, you know that it is hopeless.
Solution? Take a mini-LCD screen (½ inch or smaller), put it in a little cover, place it where you usually find the viewfinder, and the problem is solved. The cover screens the sun, and you have a working viewfinder.
In effect, the Electronic viewfinders are quite good in use, but they have all the problems a regular LCD screen has; Because the CCD chip in the camera is in constant use (to make the image on the EVF), it gets hot very quickly, resulting in degraded picture quality, and it surges battery power. Also, before a picture can be taken, the CCD has to be flushed, meaning you get a shutter lag – very, very annoying. Also, because of low resolution, the EVF system makes manual focus next to impossible. Not a good thing. Also, on still cameras, the feeling of looking 'at an LCD screen through a tunnel', rather than being in touch with your photography object, is not too comfortable.
There are still quite a few cameras who use EVF’s, but the current trend seems to be to move away from these evil devices – thank god.
On video cameras, EVF’s are in regular use – and a good thing. SLR on cameras like this is impossible (unless a pellicle mirror is used, and I have never seen that on camcorders), and the lower quality requirement for video, compared to photography, makes EVF’s an ideal solution. Many video cameras have a fold-out LCD screen as well, but the same rules for bright sunlight apply here as with digital stills cams, so the EVF is still a good idea.