Hosts

Jinchūriki ("Power of Human Sacrifice"?, English "Spiritualist Medium"), or simply hosts, are those who have had a tailed beast sealed into their bodies. Jinchūriki are created so that ninja villages can gain control over a tailed beast, having the host harness the beast's power as a weapon of battle. In most instances, they are actually stronger than the tailed beast they contain, as they have the knowledge to effectively control the tailed beast's strength. Each of the hosts that have appeared so far within the series also possess a physical characteristic that resembles their beast; Naruto for example has fox-like whisker-marks on his face, while Gaara has tanuki-like rings around his eyes.
According to members of Akatsuki, hosts of tailed beasts are typically lonely people who loathe humanity. When Akatsuki captured their first three jinchūriki, no one from their villages cared, and in fact were thankful to be rid of them. If a beast is removed from its host's body the host will die. Similarly if the host dies with the beast still sealed within them, the beasts will die too. Because of this beasts tend to be protective of their hosts, forcibly taking action when their host is in danger (as demonstrated with Gaara's sand shield). Hosts can communicate with their beast in an effort to use the beast's powers at will. Killer Bee (host of the eight tailed beast) seems to have had the most success in doing so, having a mutual relationship with his beast and being able to fully unleash the beast within him while still maintaining his sense of self. Naruto is the opposite; he is often in conflict with the fox and when he reaches four tails or more he loses complete control of his body.