To goal of every fisherman is to be able to see under water and find the fish. Alas, even having a camera down there won't necessarily do that (due to lack of light). The replacement is the crude Fish finder which sends sound waves and displays the returns. The output of these devices can be very hard to read and interpret especially if you are new to it.
In this thread, I am going to post as many useful videos I can find on this topic. Collectively hopefully they make them easier to use.
Structure Scan/Side Scan are newer technologies that use much higher frequency and narrow cone angle on the transducer to produce much sharper images of what is at the bottom or to the sides of the boat. The side images are quite useful as for the first time we can tell if a fish or structure is on the left or right. Standard fish finder shows a cone that is ever expanding from the transducer all around so you can't tell where the "target" is relative to the boat. Down scan is like a normal fish finder but higher resolution. It shows details of the bottom well but the fish looks like little dots (as opposed to "arches" in standard fish finders). This is a good video on these technologies:
This is a good video from down under explained by an American from Lowrance showing some nice images including one of crocodile! Jump to 1:30 for the real start of it:
This is HTML 5 so you can go faster and skip through intro that way.
John Easton has some really nice videos on Lowrance equipment and a nice ebook for purchase (just $5 or so). Here is a short video he shot showing a video image compared to down scan: