The difference between merge and union

The merge and union are generally the "elements" in the element class, that is to say, to operate on a certain part of the element.
Merge is the operation of the elements in the same element class, and the original elements disappear after the operation is completed.
The Union is flexible and can operate on elements of different layers, and new elements are generated in the target layer
Append, commonly used to combine multiple feature classes. such as merging data from two provinces into one feature class

As you can see from ArcGIS Help, the merge can only be done in one layer, without generating new elements, the original elements will disappear, but the nonadjacent elements can be synthesized into a plurality of elements, such as the synthesis of the Nonadjacent islands in Hawaii, if you want to separate, use the Explode tool in the advanced editing; The Union can be used between multilayer elements, which will generate new elements in the target layer; Intersect is to output the intersecting parts of two elements into the new elements.