Other forms: todays
Use today to mean this day, right now—not yesterday, and not tomorrow. If your big science project is due today and you haven't even started, you'd better get busy!
Today can also mean "at the present period of time." When someone says, "Kids spend too much time online today," they mean "nowadays," or "lately," not literally on this exact day. Before the 16th century, today was two separate words, to day, and then for another three hundred years or so, it was hyphenated: to-day. But today, we simply use today.