These are really devices that condition hard water, not soften it. Soft water refers to water from which calcium and magnesium have been removed. It is also called soft water because of the sodium in it which gives it a silky, sometimes even slimy feel. Electronic water softeners don't do it. Like magnetic softeners, they create a magnetic field around a section of the water pipe which charges water molecules and mineral ions to make them behave in a "soft" way. Please refer to the water conditioning section to see how they work.
An electronic softener is installed by coiling a wire around a section of water pipe. Some devices some themselves as a pipe through which water flows. Those, of course, are harder to install because you will need to cut a section of pipe in your home and install it there. Those pipes are also usually very narrow, reducing the flow through your pipes. They make them narrow because the magnetic field they generate is too weak for a regular-sized pipe.
Here is a good page that talks about electronic water conditioners. In my opinion, small devices like these just cannot guarantee a magnetic field that is strong enough to condition water effectively. Unless you buy yourself an industrial size device, I think you're better off getting something else. The alternative is magnetic conditioners, some of them which seem to do the job.
The 2 bigger names are Clearwave and ScaleBlaster both of which are sold at Home Depot. The comments that people leave are very mixed, usually half saying they don't work and half saying they work. I tried one and it did not do the job I expected. My preference was a magnetic softener because it could create a stronger directed magnetic field. The electronic ones produce a magnetic field by coiling a wire around the water pipe, and that cannot be directional.