Optical Viewfinder vs Electronic Viewfinder (OVF vs EVF)
As of today, all EVF's have at least a little lag. Sony, for example, make an EVF with very little lag but this is the exception rather than the rule.
In dim lighting, the EVF is easier to see though some grain/noise can be present.
The EVF has WYSIWYG (What You See Is What You Get) characteristics. Meaning images will be saved and look like what they appear in the EVF so things like exposure can be accurately set by the photographer. This cuts down on the need for bracketing.
The image can be magnified to make manual focusing easier with EVF's. Also, "focus peaking" is possible. This is a tool for manual focusing that shows the areas in focus in a different color.
An EVF (and all mirrorless cameras) has greater drain on the battery thereby resulting in fewer shots per charge.
With this finder, the photographer sees the image as reflected by mirrors onto a screen. There is absolutely no lag. This is useful for action and sports photography. This viewfinder must momentarily go completely dark while the photograph is captured. Typically, cameras with OVF's have an autofocus system separated from the sensor plane which means that images can be slightly off focus but this can be corrected with AF fine tuning for each lens. Of course, this is a hassle.
There is no histogram in the OVF and the image can be hard to see in dim lighting. A level can be present but it is not as intuitive to use as the one created by the EVF.
Most DSLR's, which use OVF's, can show the resulting image on the monitor (called Live view for Nikon) and these can often display a histogram and more intuitive level, but an EVF is more useful as the camera does not need to be held in front the photographer and is easier to use outdoors in direct sunlight.
A good, pentaprism OVF with 100% coverage costs more than an EVF to design and manufacture.