Bleeding characters take 20% increased damage from all sources. This stacks additively with itself (e.g. if there are 3 active bleeds on a character, they take 60% increased damage) and multiplicatively with other sources of damage amplification. This works with almost any source of damage there is, even unconventional ones like the passives of Red Lantern Hal Jordan (self-damage would be increased if he's bleeding), Black Adam, The Joker/Insurgency or Batgirl/Cassandra Cain, or the unique damage effects of The Ibistick or the LexCorp Set.
Characters that can cause bleed are Aquaman, Catwoman, Deathstroke/Prime, Scorpion, Wonder Woman 600, Darkseid/Apokolips and Green Lantern/Jessica Cruz Rebirth (only when coupled with 2 additional Green Lanterns).