The yarns showcased here are generally classified as Aran Weight, which is lighter than Bulky and slightly heavier than Worsted Weight. Aran Weight yarns are often considered interchangeable with Worsted Weight yarns and may have the Craft Yarn Council number 4 as well. Aran Weight yarns are popular in sweater patterns, especially with cables.
Remember that some yarns don't fall neatly into a category, so some categorized as slightly smaller or slightly larger may also meet your needs! Want more information about yarn weights? Check out this page from the Craft Yarn Council!