Chopped Clams are also known as "Minced Clams" and are exactly what you think they are: Clams that are chopped into little bits. Commonly chopped clams are made from larger clams that are tough and not usually eaten otherwise. Often these are quahogs or Atlantic surf clams. You rarely know what type of clams they are, and it typically doesn't matter.
Of course you could chop your own clams, and I sometimes do when making clams casino or baked stuffed clams, or if I dig Atlantic surf clams from my favorite beach in Scarborough, Maine, but more commonly people buy the clams already chopped/minced.
Chopped canned clams are popular as they are available almost everywhere, have a nearly infinite shelf life as far as I can tell, and are pretty good. If you live in Wichita, Kansas or Saskatoon, Saskatchewan that may be all that is easily available. A common and decent brand is Snows Chopped Clams, now owned by Bumblebee - the canning plant used to be a short walk from my place.
You can also buy frozen chopped clams. For example I recently found seawatch chopped sea clams in 51 oz containers at Sams Club. Depending on where you live you may be able to find fresh chopped clams as well. Of the three closest supermarkets to me in New Hampshire, only one carried them fresh. Often I will find them fresh as well as specialty seafood markets (one of the advantages of living by the coast!)