Shield of Sand

The Shield of Sand is the first part of Gaara's "Absolute Defence", his Armour of Sand being the second.

Overview
When Gaara was a child, his uncle, Yashamaru, told him that the Shield of Sand was the product of his mother, Karura. Karura's love for Gaara was so great that, in death, her spirit possessed the sands and used them to protect him. Later that same day, Yashamaru, under orders of the Fourth Kazekage, tried to assassinate Gaara. After his attack failed, Yashamaru told Gaara that he lied earlier: Karura did not love Gaara; she hated him. The Shield of Sand, said Yashamaru, was a result of Shukaku (sealed within Gaara) using the sand to keep Gaara, and thus itself, safe. It isn't until many years later that the Fourth Kazekage confesses to Gaara that he ordered Yashamaru to lie: Karura truly did love Gaara and the Shield of Sand truly is her doing.

The Shield of Sand reacts to any attack against Gaara, even those Gaara himself is not aware of. When he's about to be physically harmed in any way, the Shield of Sand moves between him and the threat in order to block it. Though most often used in shielding Gaara from enemies, the sand also protects him from self-inflicted injury. The Shield of Sand is automatic, requiring nothing from Gaara to use and thus meaning it can operate indefinitely. Because the shield is formed from sand that's been seeped in Gaara's chakra, it has a very fast response time, too fast for most enemies to overcome. Some, such as Rock Lee and Sasuke Uchiha, are fast enough to outpace the sand, striking Gaara before it's able to shield him; it is against opponents such as these that he relies on his Armour of Sand for defence. The combination of the armour and shield is an effective one, enabling Gaara to go years without the slightest injury.

The Shield of Sand can protect Gaara against a great variety of threats, at different times defending him from kunai, exploding tags, Senbon Shower, taijutsu, Shikotsumyaku, Explosive Clay, and Amaterasu, to name only a few. The sand itself is very good at absorbing the force from attacks, and so it is rarely even misshapen from the impact. On those infrequent occasions when attacks do push against the shield, it places enough of a buffer between it and Gaara to stop the blow before it reaches him. Only incredibly fast, piercing attacks, notably the Chidori, have been shown to actually break through the Shield of Sand and injure Gaara.

Even though the Shield of Sand ordinarily acts independently of Gaara, he is able to exert some control over it if he chooses. He can fully surround himself with the shield, eliminating the chance of it being bypassed through sheer speed. While he does this, he relies on his Third Eye to see what's going on outside, and then forms spikes from the shield to keep approaching enemies away. In Part I, Gaara usually enters this sphere of sand while preparing to enter Tailed Beast Mode.

In Gaara Hiden, Gaara uses his Magnet Release to mix "sparkling metal particles" into the Shield of Sand, fortifying it against attacks that would otherwise penetrate it. Even Karura's will sometimes influences the Shield of Sand, shaping it into her likeness as it protects Gaara to remind him of her.

Trivia

 * The Fourth Kazekage believes it is because of the Shield of Sand that Gaara was able to succeed him as Fifth Kazekage.