A widow's peak is the hairline making a V shaped point in the upper part of forehead. In extreme cases, it may go down to the upper center of the forehead - that's where it will look like a stereotypical Widows Peak where we see in Dracula and such. Some widow's peaks are less pronounced and more U-shaped, which show more only when a guy balds.
Most people don't have the stereotypical widow's peak, even though a lot of us do have a subtle one to some extent which mostly shows up in balding (for a man) or if you tie your hair back.
Those with a prominent and very obvious V shaped, widow's peak:
Jude Law, Kourtney Kardashian, Collin Farrel, Leonardo DiCaprio, William Shatner, Danny O’Donoghue (The Script), Jessie J, Wentworth Miller, CM Punk, Timothy Oliphaunt, Daniel Craig, Chris Hemsworth, Marilyn Monroe and Josh Duhamel.
Jason Statham, Bruce Willis, Jason Lee, Jean Reno, Patrick Wilson, Sting, Ryan Reynolds, Jason Isaacs, Joseph Gordon Levitt, Dwayne Johnson and the late Heath Ledger are those with a less prominent and more U-shaped peaks (which are more obvious due to balding).