They are very different fish. Somewhere along the lines someone got the names mixed up and it stuck. But they're using the names incorrectly when they're saying they're going "pickerel fishing". Here's some photos to clearly illustrate the difference.
The above is a WALLEYE
The above is a PICKEREL (this one is a chain pickerel)
that bottom is a grass pike
Here's the best explanation I could find. Some of it comes from Wikipedia.
The walleye or yellow pickerel or pickerel is a freshwater perciform fish native to most of Canada and to the northern United States. It is a North American close relative of the European pikeperch. The walleye is sometimes also called the yellow walleye to distinguish it from the blue walleye.
In some parts of its range, the walleye is also known as the colored pike, yellow pike or pickerel (esp. in English-speaking Canada), although the fish is related neither to the pikes nor to the pickerels (which are actually a species of pike called the chain pickerel), both of which are members of the family.
In Southern Ontario the name Yellow Pickerel has been around for a hundred years. People dropped the yellow and call all walleye pickerel.
The fish is a Walleye but the habit of calling them Pickerel is handed down from each generation to the next.
Hey guys, got a fish ID question for you. The picture below is a friend of mine who I was fishing with a couple weeks ago at a private lake owned by my family. I fish this lake quite often and have caught one of these fish before, but it's probably been 10 or 12 years. We eventually came to the conclusion it looked most like a grass pickerel, but as time goes by and I see more pictures of grass pickerel, I am less convinced. Can anyone come up with the actual species for me? Thanks.