How does ultrasound work? How is the signal processed? Why is ultrasound 2D? Is there a way of making ultrasound 3D without piecing together 2D? How close is ultrasound to sonar? 
 A: ultrasound is basically sonar, only engineered for imaging very small objects at very close range instead of very large objects at very large range.
As ultrasound penetrates tissue, some of it is scattered and/or absorbed and does not bounce back towards the receiver. Of the amount which does bounce back, some of that is absorbed and/or scattered on its way back as well. 
These effects place natural limits on the amount of accurate 3-D information which can be deduced from conventional ultrasound when you are imaging wet, live tissue. 3-D information can be deduced more easily when you are imaging, for example, an aluminum casting to detect inclusions or porosity. 
A: Conventional ultrasound sequentially transmits roughly 128 focused 'pencil beams' with very short pulses in a fan shaped 2D pattern. After each pulse transmission it samples the echoes as a function of time. Using the propagation velocity each echo can be associated with a specific point in the imaged object (re. round trip time of flight) so each pulse provides a single line of imaged points from the object.  The 128 pulses are repeated every 33 ms to provide a real time 2D image.
1) Conventional Ultrasound is 2D because only 128 pulses can be transmitted in 33 ms because of the round trip time of flight.
2) Google 'Ellipsoidal Backprojection' for a method of getting real time 3D images in ultrasound.  In theory, a single unfocused pulse can provide a 3D image using this approach.  The math behind it is more involved. It is roughly a synthetic aperture approach.
3) At the basic level ultrasound and sonar are the same.  But the geometry and other parameters are very different--eg. the distances are much greater in sonar, which means the time of flight is an even greater limitation.
