A Parallel is simply a specific type of Insert. When the site was first founded, there was no specification for "Parallel", so all sets that were added within the first couple years of the site's launch needed to be manually moved from "Insert" to "Parallel". Not all sets were handled. There probabbly aren't many more, but it wouldn't surprise me if there were still a random few out there still listed as "Insert".
-- Dan --
Note: Please see my profile for more info regarding trading (section updated 3/4/2024). I have added a large portion of my inventory to the site, and currently have trading turned on (details are in my profile).