In the standard Hertzsprung-Russell diagram, blue giants are found on the upper left corner, thanks to their high luminosity and early spectral type. They are actually post-main sequence, and burn helium rather than hydrogen.
Blue giants are extremely luminous, reaching absolute magnitudes of -5, -6 and even higher. Their temperature is high enough (20,000 K or more) that a sizable fraction of their energy output is in the ultraviolet range, thus invisible to our eyes.
Most stars of this types are found in O-B associations, large collections of loosely bounded young stars. Since they are so massive, their expected life is very short (in the order of tens or hundreds of million years), and current theories predict that most of them will end their life as supernovae.