That’s the big question isn’t it? We are fortunate enough to be able to ask it, and that alone is pretty awesome. Human beings are likely the only life forms on earth, and possibly the whole universe, that can actually ask that question. So just having the capacity to inquire about the nature of our own experience is rather special.
Other beings may have a feeling of questioning their existence, but as far as we know humans are the only life forms that use language in such an advanced way to rigorously inquire into the nature of our being. But language can be problematic, since communication is based on shared understanding of the words we use, and can lead to confusion (and worse) when the meaning of words isn’t clear.
To talk about consciousness I’ll need to define exactly what I mean when I say consciousness. The word gets used in a variety of contexts, so being clear will help avoid confusion. I am going to use a rather broad definition. When I use the word consciousness I am simply referencing the ability or capacity to have an experience.
An experience can be sensory, such as seeing the color green, or feeling the wind, or hearing a dog bark. But an experience can also be the feeling of an emotion, such as joy or frustration, a pang of hunger or a wistful memory. Even the thoughts you’re having while reading this are something you are experiencing.
In philosophy they often use the word qualia to mean a single experience, such as the experience of the color red, but I find the term a bit reductionist. You can never experience the color red exclusively on its own. You will see the texture of the surface, and highlights and shadows suggesting shape and volume, and the contrasting background of other things nearby.
Even if you were in a virtual reality rig with a uniform display of pure red pixels, you would still have the other sensory experiences, hearing, smell, touch, as well as the interior experience of thoughts and feelings.
So consciousness in humans is all of those things and more. In other living things consciousness will have a different expression, but it will still be there. Your family dog, if you have one, has a sensory experience where scent is paramount and vision is monochromatic. Your goldfish’s experience will be different too, a little less expanded, being trapped in a bowl and all. Even the fly buzzing around has a limited scope of experience, but is still savvy enough to avoid getting swatted. The houseplants turn and grow towards the sunlight. It’s possible to imagine even the mildew on the bathroom tiles has some sense of when it’s wet or dry.
In this definition of consciousness every life form on earth, and elsewhere, will have some form of consciousness, some experience of what it is like to be that being. But is consciousness exclusive to living things?
On Mind and Matter 
Very interesting, the conceptual platform I use to think about consciousness is duality-nonduality. I’m talking here about conscious duality-nonduality embedded in cognitive processes, the extent to which we maintain conceptual separation between phenomenon. As you note, most organisms are aware of stimulus or input at some level. But the nondual perspectives, I think, denotes what most of us understand by consciousness.
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