Transducers come in various frequencies. Low frequencies generally range from 50 to 100 KHz. High frequency models range from 180 to 200 KHz. Lower frequency sound waves can travel greater distances. They penetrate further in the water.

Lower frequency = greater depth.

IMPORTANT: If you purchase a transducer separately, or you already have one, make sure your model will work with the frequency guidelines of your fish finder. This applies for portable fish finders as well as mounted models.

Dual frequencies

A transducer with dual frequency is probably the best bet for accurate fish readings. Dual frequency (or beam) models, as the name implies, have two different beams that project at different frequencies. One is set to a low frequency and the other to a high frequency.

Many fish finders will allow the fisherman to switch between the two beams on the display. There are also models that have a split screen display that allow the angler to see images from both beams separately at the same time. Some manufacturers even have displays that combine the images from the two beams into one image. The display essentially combines the high-frequency detail with the wider beam and deeper search capabilities of low frequency. 
For deep water fishing, dual frequency transducers are a great option. They let you find the general area of the fish over a large volume of water using a low frequency-wide beam setting, and then, for greater accuracy, pinpoint the depth and location using a high beam-narrow cone setting for greater clarity. 
For shallow water fishing (i.e. Rangi channel), a dual frequency transducer may be overkill. There is little need to see fish at a depth greater than a hundred feet so a single beam, high frequency model may a better option.

Cone Angle

Cone Angle becomes important because the greater the angle the bigger the coverage area and therefore the bigger the area that the fish finder can 'see'. As you move further away from the centre line, the clarity of the image decreases. 
A large cone angle ranges from 40 to 60 degrees, has a larger coverage area but provides less clarity at greater depths. 
A narrow cone ranges from 15 to 30 degrees, has a smaller coverage area but provides more clarity at greater depths.

Frequency and Cone Angle Working Together

High Frequency transducers come with either a narrow or wide cone angle. For shallow water fishing, a high frequency (180 - 200 KHz), narrow cone angle (20 - 30 degrees) model may be a good choice. You will not be able to see great depths, but you're in shallow water. The narrow cone angle will allow you to see things more crisply than a wide cone and will allow you to be able to distinguish the objects you see on the display easier. If you really want to see a large area, then think about a wide cone angle instead. 
For deep water fishing you will most likely want to search the highest volume of water possible in the shortest amount of time. Consider fish finders that have a low frequency (50 - 100 kHz) for greater depths, and a wide cone angle (40 - 60 degrees) for greater horizontal distances from the cone centre line. This combination will allow you to see at greater depths, but the definition (how crisp the image is on the fish finder display) will not be as good. You can increase the definition by buying a fish finder with a higher sensitivity display. 
Low frequency, wide cone angle setups do have some limitations. For example, you can cover a wide area and a greater depth which gets you into the 'general' area of where the fish are, but we don't want to fish in the general area, we want to put our lines directly in front of the fish.

Read about Transducer Mounting Systems...