Mechanical shutter vs electronic shutter
A few DSLRs – notably the EOS 90D and EOS-1D X Mark III – have electronic shutters in addition to mechanical shutters, but usually DSLRs have mechanical shutters. All of Canon's EOS R System mirrorless cameras, on the other hand, have both types of shutter.
With an electronic shutter, the mechanical shutter is locked open and the imaging sensor is essentially read electronically. The biggest advantage of this is that shooting can be totally silent – not just quieter, with the shutter damped or suppressed as it is when you select the S ("soft" shooting) option available on some cameras, but completely noiseless. This can be invaluable when you're photographing nervous wildlife, slumbering babies or the quiet parts of weddings, for example.*
Electronic shutters also enable faster continuous shooting. "With mechanical shutters," Mike says, "the fastest possible in our DSLR cameras was 16fps [frames per second] on the EOS-1D X Mark III, a high-level professional camera. With its electronic shutter, the EOS R10 can give you the option of up to 23fps for a fraction of the price, while the EOS R7 can shoot at 30fps, matching the speed of the EOS R3, another professional camera, but for a third of the price."
However, the phenomenon of "rolling shutter" can be a problem when you photograph action with an electronic shutter – fast-moving objects such as a swinging golf club or a rotating propeller can move in the time it takes for the whole frame to be read, leaving them distorted in the resulting image. The phenomenon is greatly reduced in the latest mirrorless cameras, but mechanical shutter is recommended for shooting fast-moving action.
Shooting with electronic shutter in some artificial lighting, notably fluorescent lights, can be problematic if the light is out of phase with the shutter. For similar reasons, using flash with electronic shutter can result in uneven exposure across the frame.