Both beer yeast and bread yeast are products of a single-celled fungus called Saccharomyces cerevisiae. But they are formed from different strains of this species. The main difference between beer or brewer's yeast and baker’s or bread yeast is that brewing requires yeast that can produce CO2 and alcohol whereas baker’s yeast has a much stronger focus on the CO2. Bread yeast is primarily used as a raising agent whereas brewers expect the yeast to be virile and reproduce at a fast rate that will transform the sugary wort into a drinkable beer.
There are plenty of examples of these two being used interchangeably. However, each comes with it's own precautions and aspects to be taken care of.