Aposematism is a form of animal signaling that is essentially the reverse of camouflage. Instead of trying to hide, an animal will actively try to make itself as visible as possible. Which sounds extremely maladaptive but it's done with purpose--namely to advertise that attempting to eat them will usually end up very, very badly for the predator. Some common examples: Poison dart frogs will kill you, skunks are noxious, wasps sting the shit out of you, and honey badgers will just fuck you up.
Aposematic animals often have bright, contrasting colors and patterns. This is why aposematism is also widely known by the less technical term 'warning coloration' (though not all signals are visual). Alternating colors are particularly common as are yellows and reds. It is no coincidence that most human caution signage uses these same patterns--millions of years of evolution has taught us that they represent danger.
The classic example of aposematism is poison dart frogs (as above), but also includes blue-ringed octopuses, butterflies (and caterpillars), bees and wasps of all kinds, spiders (like tarantulas, huntsmen, and orb-weavers), coral snakes, sea snakes, rattlesnakes, and pretty much anything that lives in Australia.
Because it's energetically costly to put up such a strong defense, many animals will simply copy another species' signals without actually being dangerous. This is called Batesian mimicry. There are hundreds of species of insects that imitate bees alone. Some even free ride on others of their own species. Monarch butterflies, for instance, don't produce poisons on their own but instead concentrate them from their diet as caterpillars and so aren't always poisonous. Nevertheless, as a species all monarchs benefit from the danger that only a few may actually represent.