As described by the National Institutes of Health, there are three forms of mercury that can be potentially deadly: elemental mercury, inorganic mercury and organic mercury.
Elemental mercury, which is what you find in glass thermometers, older dental fillings and florescent light bulbs, is harmless to the touch, but can be fatal if inhaled. Even if the person exposed survives, poisoning can still lead to long-term or even permanent lung and brain damage.
Inorganic mercury, which is used to make batteries, can be deadly when ingested, and lead to kidney damage and worse.
Organic mercury, found in fish, can be inhaled or ingested, and usually only affects those exposed over the long term, except in rare cases. Symptoms can range from memory loss to blindness to seizures and more.