A host is an animal that has undergone the Hosting Process, most commonly a human. Once the Hosting Process is complete, the creature inside of the Host will be able to utilize its body and fully control it. There are certain abilities that Hosts have that the basic creatures do not (mainly transformation, speed, and the ability to call out to other Hosts). Our kin can bend the body of the Host and shift it into other things, appearing to be other creatures or manipulating the basic body to appear or work differently. Within animal hosts, this is limited. However, within a human host they can transform into anything. There are certain defects within a host - twitching, altered voice, and glowing eyes in the dark. However, these are all lessened with experience within that Host and form.
Also see: Host Surfacing.