What happens if you block an attacking creature with a creature that has equal power and toughness as the attacking creature? Do they both die or bounce off each other?
It will depend on their actual power and toughness. If their power is higher than or equal to their toughness (two 2/1 creatures or two 1/1 creatures blocking each other) they'll both die; if the toughness is higher (two 1/2 creatures) they'll both survive.
That's the story for vanilla creatures (i.e. without abilities); some abilities, like First strike and Deathtouch, might influence the outcome.
A combat isn't "won" or "lost". In fact, creatures aren't really directly compared at all in combat. Each creature deals damage equal to its power to the other creature, and then, if they have received damage equal to or greater than their toughness (including damage from other sources earlier in the turn), or any damage at all from a creature with deathtouch, they are destroyed.