Does an effect that says 'draw a card' replace the normal draw in your draw step or add an additional draw?

The normal draw in your draw step is a game rule defined in CR 504.1: each player draws a card at the beginning of their draw step. This is a mandatory game action, not a triggered or activated ability that can be swapped out by default.

When a card or effect simply says 'draw a card,' it creates an additional card draw on top of the normal draw step draw. The two draws are completely separate events. For example, if you have a Howling Mine on the battlefield, you draw your normal draw-step card first, then Howling Mine's triggered ability resolves and you draw one more — netting two cards total.

The only way an effect replaces your draw-step draw rather than adding to it is if it uses explicit replacement (or skip) language. For instance, an effect that says 'Skip your draw step' (CR 614.10) or a replacement effect worded as 'If you would draw a card, [do something else] instead' (CR 614.1a) would modify or eliminate the normal draw rather than supplement it.

A concrete example: You control Divination's effect resolving on your opponent's end step, drawing two cards. On your next turn, you still draw your normal draw-step card — Divination did not consume or replace it. You simply drew two extra cards earlier.

In summary: 'draw a card' means draw an additional card. Only 'skip your draw step,' 'you don't draw a card this turn,' or 'instead' language can replace the built-in draw-step draw (CR 504.1, 614.1a).