Generally, you have to check a box at the start of the game to record it.
It then saves the replay until you delete it. While watching it, you can play, pause, rewind or fast forward and it marks the elapsed time. You can choose which parts of the map you want to see (all uncovered or only the player you follow), and can even follow a certain player's actions.
That's how it generally works. Might be different in EaW.
~Dark Cupcake of the Sith