Sentience is generally taken as being self-aware, that is, being aware of yourself as an individual different from others in your group. You can feel/emote, and perceive abstract notions, as well as being able to anticipate and, most importantly, think. This leads to what some say is the ultimate proof of human intelligence - the ability to create, whether it be an atomic bomb or a wonderful piece of art.
Animals do not seem to think, but seemingly can can feel emotion and the most intelligent may even be aware of themselves as individuals. Cows being led to slaughter have been discovered to be crying, aware of their impending deaths. Chimpanzees can be taught sign language, and have used signs to communicate their feelings. Crows can count. Dolphins recognise themselves in mirrors.
Do we share the Earth with all its other forms of life, or is it right that we dominate it?