It's tricky to put updefinitions of these things. Even speciation.
Because we know that life is essentially one long continuous fluid dynamic. You can look at an example of a species 10,000 generations removed and say, OK, this is definitely different than that. But if you look at your own kids, there isn't a whole lot of difference. Or even a few thousand years ago. There doesn't seem to be much difference. Where you draw the lines is somewhat arbitrary, but once the lines are drawn you can exclude things which are difinitely not that, and things which definitely do qualify.
For instance, if you define life as:
Living organisms undergo metabolism, maintain homeostasis, possess a capacity to grow, respond to stimuli, reproduce and, through natural selection, adapt to their environment in successive generations.
Then you have set up defining characteristics which must be met to satisfy that particular definition. This excludes the virus because it lacks homeostasis and other elements, yet it does reproduce and can be "killed". Prions are interesting as well. Not living in any sense, yet they hyjack a cell's biology like viruses do, and in the end are capable of reproducing themselves.
http://en.wikipedia.org/wiki/Prions
That is even more of a grey area. But what we end up seeing from this is a kind of sliding scale of complexity leading toward life. Not to claim that we are the descendants of Prions, but what that represents is non-life behaving in some ways like life.
We define the linees in the sand and determine what will be the in-group and out-group, just as we do with our notions of family. But in the same way, we are interconnected to every human on the planet through heredity, and eventually that lineage can be traced back to completely inorganic primordial photons rattling around after the big bang.
It's all about where you draw the lines and what you are trying to select. You can put a circle around birds and definitely exclude the Komodo dragon, even archeopterex, but you couldn't then deny that archeopterex doesn't have many bird-like qualities. The bar is set at sharing all the qualities that define a group which are found in no other.
So viruses are excluded from life despite sharing some similarities the same way that humans are excluded form birds despite sharing bilateral symetry, craniums, brains, spinal chords, tetrapoidal skeletons, lungs etc...