Sonar is a general term for technology using sound pulses (pings) sent thru the water column to bounce off of objects in the water and then returned to the unit to show on the display as "Echo returns"...
There are 3 technologies in the general scope of Sonar...
This is 2d Sonar..(2dimensional)..it is created by sending many successive sound pulses (pings) thru the water column in a "cone shape"...and creates a "cartoon-like" display image of echo returns detected under the transducer in that "cone shape"...
This is DI (Down Imaging)...it is created by sending many successive sound pulses (pings) thru the water column in a "thin slice" shape...and creates a "picture-like" display image of echo returns detected under the transducer in that "thin slice" shape...
The 3rd Sonar technology is SI (Side Imaging)... Of which you did not ask so we will leave that for a different discussion...
Rickie