In modern English usage, being means conscious entity. (See also I think, therefore I am.) While this clearly includes all animal life and any spirit beings that might exist, there is currently no conclusive scientific evidence as to whether or not plants, minerals or viruses are conscious.
In the more general philosophical sense, a being is anything that can be said to 'be' in various senses of the word 'be.' 'Be' is a word, like many other words, that has different senses. So there are different senses of the word "be," and accordingly, one might say, there are different "kinds" of beings, or ways of being.
Some philosophers deny that the concept of ""being"" has any meaning at all, since we only define an object's existence by its relation to other objects, and actions it undertakes. The term "I am" has no meaning in the English language; it must have an action or relation appended to it. This in turn has led to the thought that ""being"" and nothing are closely related, developed in existential philosophy. Some modern theories in physics draw parallel conclusions, based upon ideas of matter popping in and out of real space.
For philosophic consideration of the nature "being" in a verbal sense, see ontology.
This article clearly needs a lot of work...philosophers, please help!