A union combines two lists into one containing every distinct item from either, with duplicates collapsed.
What union means here
Everything in list A, everything in list B, each appearing once. Items present in both appear a single time in the result.
The everyday case
Keyword exports from two tools. Backlink lists from two sources. Page inventories from a crawler and an analytics platform. Anywhere the same subject was measured twice and you want the combined picture.
The other two set operations
Intersection keeps only what appears in both. Difference keeps what appears in one and not the other. Union, intersection and difference answer three different questions about the same pair of lists.
Try it: Merge Two Lists (Union) on SeoWolf's Notepad.