A collection keeps units distinct and recognizable, like pieces of fruit on a platter. It can combine any CC licensed content so long as license requirements are followed. Examples of collections are a book of poems, a collection of musical scores, or an album of images. A collection may have unique content in a forward and/or afterward which is newly licensed, but the previously existing content retains its existing licenses, which still apply.
A remix if s form of derivative combines elements from multiple sources into a new form which is sometimes unrecognizable to the originals. Although a remix is based on previous works it is a new creation which is newly licensable. However the original licenses must still be compatible with the new use. For instance an ND license would not allow any remix/derivative to be created from it. Examples are a translation, a film version from a novel, or a photo montage.