Because of their small size and unique metabolism, cats (especially) and dogs tend to be highly sensitive to poisonous plants. Many toxic substances require quick home treatment followed by immediate veterinary care. Veterinary follow up is critical to prevent secondary effects of the poison. A veterinarian can also monitor the pet for complications.
t should be said that most cats will not eat plants outdoors, as long as grass is available. However, occasionally, out of boredom or curiosity, a cat may nibble a leaf or stem.
Still, if you're of the "safe rather than sorry" frame of mind, as I usually am, you need to know which plants are potentially harmful, and by what degree.