Thumb

In human anatomy, the thumb is the first digit on a hand.  The human thumb is fully opposable to the tips of the other fingers in that it may position itself, and be folded inward, toward the rest of the hand and fingers, if so required. It rotates at the carpometacarpal joint and so can complete the sometimes quite delicate task of grasping objects by pressing them against the rest of the hand or finger(s). 

Anatomy of the thumb
Bones

The thumb consists of 3 bones:
 distal phalanx (of the first digit)
 proximal phalanx (of the first digit)
 first metacarpal

Muscles
Its movements are controlled by eight muscles (each with "pollicis" in the name):

In the forearm
 extensor pollicis longus
 abductor pollicis longus
 flexor pollicis longus
 extensor pollicis brevis

The extensor pollicis longus tendon and extensor pollicis brevis tendon form what is known as the anatomical snuff box (an indentation on the lateral aspect of the thumb at its base) where one can usually palpate the radial artery.

In the hand
 abductor pollicis brevis
 flexor pollicis brevis
 opponens pollicis
 adductor pollicis

The first three of these form the thenar eminence.

Grips

Typical interdigital grips include the tips of thumb and second finger (forefinger/index finger) holding a pill or other small item, or thumb and sides of second and third fingers holding a pen or pencil.

Origin of the thumb

The evolution of the opposable or prehensile thumb is usually associated with Homo habilis, the forerunner of Homo sapiens. This, however, is the suggested result of evolution from Homo erectus (around 1 mya) via a series of intermediate anthropoid stages, and is therefore a much more complicated link.

The most important factors leading to the habile hand (and its thumb) are:
 the freeing of the hands from their walking requirements—still so crucial for apes today, as they have hands for feet, which in its turn was one of the consequences of the gradual pithecanthropoid and anthropoid adoption of the erect bipedal walking gait, and 
the simultaneous development of a larger anthropoid brain in the later stages.

The opposable thumb has helped the human species develop more accurate fine motor skills.

Other animals with opposable thumbs

Many animals, primates and others, also have some kind of opposable thumb or toe:

 Bornean Orangutan - opposable thumbs on all four hands. The interdigital grip gives them the ability to pick fruit.
 Cebids (New World primates of Central and South America) - some have opposable thumbs
 Chimpanzees have opposable thumbs on all four hands.
 Koala - opposable toe on each foot, plus two opposable digits on each hand
 Opossum - opposable thumb
 Giant Panda - Panda paws have five clawed fingers plus an extra bone that works like an opposable thumb. This "thumb" is not really a finger (like the human thumb is), but an extra-long sesamoid bone that works like a thumb. 
 The 4-toed sloth - however, the related 3-toed sloth does not.