Scuba Diving V/S Snorkeling

Scuba diving is a mode of underwater diving in which a scuba diver uses a self-contained underwater breathing apparatus (scuba) to breathe underwater. Unlike other modes of diving, which rely either on breath-hold or on breathing gas pumped from the surface, scuba divers carry their own source of breathing gas, usually compressed air, allowing them greater freedom of movement than with an airline or diver's umbilical and longer underwater endurance than breath-hold.
Snorkeling is swimming with a snorkel (a mask and a tube) that allows you to breathe through your mouth when floating underwater near the surface of the water.
Comparison
Purposes- Recreational purposes of scuba diving, include cave diving, wreck diving, and ice diving. Professional purposes include civil engineering, underwater welding, offshore construction or for military purposes. Recreational purposes of snorkeling include observing fish and algae and coral reefs especially in water bodies with minimal waves and warm waters; also interesting things to see near the water surface.
Technique- While scuba diving, the swimmers entire body is under the water. The diver's nose and eyes are covered by a diving mask; the diver cannot breathe in through the nose, except when wearing a full face diving mask, but adapts to inhaling from a regulator's mouthpiece. While snorkeling, head & nose of the swimmer is underwater. The snorkel tube can flood underwater. The snorkeler expels water either with a sharp exhalation on return to the surface or by tilting the head back shortly before reaching surface.

Underwater duration - The swimmer can stay under water longer as one does not need to hold ones breath. The swimmer needs to hold breath to swim under the surface of the water.

Equipment- For scuba diving following equipment are used: Pressurized gas tank strapped to the back of the diver, single hose, open-circuit 2-stage diving regulator with the first stage connected to the gas tank and the second to a mouthpiece, swim fins attached to feet, diving suit. For snorkeling following equipment are used: Diving mask, L or J shaped tube with a mouthpiece at the lower end and sometimes swim fins attached to feet.

Training- Requires training in how to use the breathing equipment, safety procedures and troubleshooting in scuba diving. Although no centralized certifying or regulatory agency many dive rental and sale shops require proof of diver certification. Snorkeling requires no training. Snorkelers favor shallow reefs ranging from sea level to 3-12 feet. Deeper reefs are also good, but repeated breath holding to dive to those depths limit the number of practitioners and raises the bar on fitness and skill level.