Personal Flotation Devices - "Life Jackets" and Throws
They're what float; you don't!

Life Preserver Requirements

A Coast Guard approved, wearable life jacket - also referred to as a personal floatation device (PFD) - is required for each person on board a recreational boat, including canoes, kayaks, and personal watercraft (PWC). A person being towed by a boat, such as on water skis or knee board, is considered a person on board, and an approved life jacket is required either on the person or on board.

Type I - Offshore

For all waters; especially offshore remote waters where rescue may be delayed. Turns most users face up.

Adult: 22 lbs. buoyancy.
Child: 11 lbs. buoyancy.

Type II - Near-Shore

For protected waters - near shore - where fast rescue is likely. Turns some users face up.

Adult: 15.5 lbs. buoyancy.
Child: 11 lbs. buoyancy.

Type III - Flotation Aid

For calm, protected, inland, near shore waters where there is better chance of fast rescue. Users must turn themselves face up.

Adult: 15.5 lbs. buoyancy.
Child: 11 lbs. buoyancy.

Type IV - Throwable Devices

Not to be worn. For throwing to a person in the water to grasp and hold until rescue.

Type V - Special Use

Use according to label and outside other clothing. For special uses and conditions; label states limits of use which may include approval only when worn.

Inflatable Device

Many types now USCG approved. Use according to label and outside other clothing. Approved only when worn, Inflatable PFD's are more comfortable to wear which encourages wearing at all times. 

Manual inflatables are usually classified as Type III PFD's
Manual/Automatic as Type V with Type II or Type III performance.
When inflated they provide superior buoyancy than traditional jackets, 22.5 lbs. buoyancy for inshore types, 35 lbs. buoyancy for offshore types.