In typesetting, the term justified refers to how words line up with the margins of a page or column. Whether you choose to make the text left, right, or fully justified depends on the purpose of the text.
If you're typing a letter, you may want the date and your address to be right justified. The last character on each line would touch the right margin. The salutation (the part that says, "Dear Sir or Madam") would be left justified, with the D touching the left margin. The body of the letter could be left justified, with the right margin uneven, or fully justified, with both the right and left aligned.