=TODAY() returns today's date and =NOW() the date and time. They take no arguments, but the brackets are required.
Both recalculate every time the sheet does, so a formula that uses them is always current: =C2-TODAY() counts the days left until the date in C2, and tomorrow it shows one fewer. That also means the result changes from day to day, which is what you want for deadlines and ages, and what you don't want for a record of when something happened. To stamp today's date permanently, type it, or press Ctrl+;.
Typical uses: =IF(C2<TODAY(),"Overdue",""), and age in whole years with =DATEDIF(B2,TODAY(),"Y").