In Japanese, the word "panko" simply means breadcrumbs. In the U.S., though, panko refers to a particular type of Japanese breadcrumb. Panko breadcrumbs are dry. They have a shard-like shape and a uniformly light color (most panko is made from crust-free bread). Their unique texture is particularly well suited for breading and frying producing a light, crunchy, delicate coating. They seem to absorb less oil and stay crisp longer than regular crumbs. They also make a great topping for casseroles.
Though the texture won't be exactly the same, you can substitute regular dry breadcrumbs.
Panko used to be hard to find anywhere other than an Asian market, but recently we've noticed it cropping up in large supermarkets. Look for it in the Asian foods section, the dry breadcrumb section, or near the fish counter.
Store well sealed in a cool, dry place.