What is intelligence?
You could say (and many have) that intelligence is that which is measured by intelligence tests. The problem with this is that it assumes the answer to the question it is asking. If you are dealing with normal humans, this isn't too much of a problem, but for anyone outside of the normal group the tests don't really work.
Animal intelligence certainly exists, with 98% of the genes between humans and some apes being identical, it would be the height of hubris to suggest it didn't, but you can't measure it with IQ tests. However some chimps (washo and panbanisha spring to mind) can communicate using sign language. Unfortunately, they only say simple things like "want banana" of "jenny hug", much like a very young child might.
Russian Artificial Intelligence (AI) researchers say that someone or something is intelligent if it gives intelligent answers to intelligent questions. While there are other definitions, this one does get around the problem that western AI have of arguing about what is intelligent and what isn't (for example: searle's chinese room). It also means that you can avoid the western split between social AI which is about how good the man-machine interface is, and technical AI which is more about things like does your house design program tell you if you are putting a door where you can't open it.
You also have extra-terrestrial intelligence. While they would have to be intelligent to communicate with us, the odds of them being anything like us is extremely remote, and so their intelligence would seem strange and very alien to us. Having said that, even people from different ethnic and cultural backgrounds seem alien in their intelligence, so as long as we are prepared to work at it, it is perfectly possible to have some area of common understanding.
So what is intelligence? The Russian AI researchers probably have the most useful definition, but do make up your own mind.
Part of the Intelligence Project