Blood is a bodily fluid in humans and Night People that delivers necessary substances such as nutrients and oxygen to the cells and transports metabolic waste products away from those same cells. It varies from species to species, and possess different qualities.
Human blood serves as a source of sustenance to vampires, due to its ability to carry oxygen (which vampire cells cannot). It also has more mystical qualities, presumably its ability to strengthen a vampire's Power.
Unlike other races, vampire blood cannot carry the oxygen needed for respiration. For this, reason they need to regularly ingest human blood. Vampire blood is also considered "poisonous" in the sense that in large quantities, it destroys human cells and propagates the change from mortal to made vampire. However, it has enhanced regeneration abilities that can heal wounds at an accelerated rate. Vampire blood is also noted to have a unique taste that is found to be "delectable" to certain mortals.
No distinction has been made for witch blood, but it appears to have the same physical properties as that of a human and can be used a substitute for feeding by vampires.
No vampire has been seen feeding on werewolf blood thus far, but it is implied that it is sustainable to vampires. 
The nutritionous capability of shapeshifter blood is debatable, though since vampires can feed on animals, and shapeshifters can transform into animals, there is a possibility that their blood can be ingested by vampires. This is further supported by the fact that werewolves can make blood donors (werewolves are a subset of shapeshifters).