The Search By Tags field allows you to search for one or more tags. Clicking on Harmon in the Browse by Tag will yield the same result as putting in Harmon in the Search By Tags field, but you can put more than a single tag in the box. If you were to put in Harmon and DPA-1a, you would be looking for all pictures taken at Harmon that had EMD E7 A units.
Search Items allows you to specify fields that you are interested in searching on. You can use a combination of fields and tags. The specifics on searching items can be found in the Search Items help page.