What is meant when scientists speak of not finding life "as we know it"?
With astronomers constantly finding more and more extrasolar (outside of our solar system) planets, scientists have been looking once again at the mathematical and scientific likelihood that there is life of some kind on other planets.
Despite popular culture's interpretation of what that life might look like, scientists know all too well that any extraterrestrial life we discover could well be very different from what we have here on Earth. It could be at the cellular level, it is likely to be some kind of plant form, and unlikely to be either intelligent or to have any human-like features.
In short, we know "life" only from our own definition of it here on Earth. If we ever find other life, it is unlikely to be similar to us.