Underlining and Italicizing
Underlining words and Italicizing words in standard written English mean the same thing.
Handwriting and typing normally show underlining. Typesetting for print usually uses italics. Most computers can go either way.
Whichever way is chosen, be consistent and keep the same style throughout.
Grammar Slammer normally uses italics because of the Web file convention of using underlining to show a jump spot or link.
Underlining Foreign Words or Abbreviations
Underlining Words for Emphasis
Underlining Items Which Name Themselves
Titles Which Take No Punctuation