From the documents or video's that are available that I've seen from people who've allegedly had 'contact' with another race of beings, there were some commonalities.
Of all the video's from those who've claimed to have had some contact with an extraterrestrial, all the alien's have a common feature. That is, in each of the cases, the 'being' is a humanoid. Meaning they have two arms, two legs and a head.
I recall one video from the past (forgot the name of the video) where one person who claimed to be a witness or worked in something related to the government. He said that when the government scientist's were shocked to find that nearly all the races of beings out there are humanoid in nature. This caused a big shock to them (the scientists) because they were expecting life forms to have evolved differently on other planets like earthforge has mentioned.
If I recall correctly, that person mentioned that the government (U.S.) / military, were particularity frightened of a race of being's that were identical in appearance to that of humans here. I think they were blonde haired, blue eyed folks. Meaning they could blend in to the population and never be noticed.
Now whether this person was telling the truth or not is another matter altogether but it is pretty interesting.