Underline and underscore both means emphasizing but underscore is used in American English.The third word may be highlight.They seemed to me interchangable but lets wait for a native speaker answer.You know in English some words are used differently though they have the same meaning
Hi spinnaker, if you are asking about the ASCII symbol, then it should be "underscore" ---> _ But when you see a word like this ---> word ----> that word is "underlined" (maybe also "underscored" in American English, but I think the usual word is still "underlined")