^Compared to some bizzare entities one could encounter in works of science fiction, those look and behave pretty ordinary.
There are two schools of thought on the nature of aliens. The first assumes, since laws of physics apply to the entire observable universe (after decades of observation we can be confident in this), that for each problem there must be a solution that is vastly simpler than all other solutions for that problem. Colloquially, it means that certain designs will be vastly more prevalent than others. Vastly can easily mean such a high number that a design might never appear spontaneously during the universe's entire existence. A prominent representative of this school would be Marvin Minsky. In his paper (highly recommended reading)
Communication with Alien Intelligence he uses artihmetics as an example of a number system for which no alternatives could be devised, because they either don't exist or are so complex that nobody figured out one yet. He also runs a technical experiment that shows that before a complex machine is devised for solving a problem (a B-machine), a simple machine that solves the same problem (an A-machine), will appear many times, again and again. So it is much more likely that the A-machine will be used, and nobody will bother searching for the B-machine.
He calls this the "sparseness principle". Using the sparsness principle, and what we know about chemistry and astrochemistry, we may claim, in decreasing order of certainity, that life requires an external source of energy (this can be safely considered to be completely certain; an alternative can't be imagined under the known laws of the universe),
that it requires catalysts (also almost completely certain, but based on observations; in principle, there could be an alternative solution that still obeys the laws of the universe, but it is not likely),
that it needs liquid medium,
that it must be carbon-based,
that the liquid medium will be water,
that it will of consist of large repetitive structures (nucleic acids, proteins, lipids, large carbohydrates) made out of a few different simple elements (nucleotids, amino-acids, fatty acids, alcohols, simple carbohydrates),
that it propagates through a self-replicating machine (either its own or someone elses),
that the machine must have three basic parts: the blueprint (DNA), the replicator (RNA, proteins) and a membrane that protects the blueprint and replicator (lipids)... And so on. The ultimate result of using the sparseness principle would be that the thought processes of an intelligent alien must be similar to the processes of a human, at least on a very basic level, and so communication will be possible, provided that the parties want to communicate.
The other school assumes that the sparseness principle is false, that there are many solutions to problems, and it ultimately follows that aliens must be so different from us that communication, or understanding at any level, will be impossible. Prominent representatives of this school would be Stanislaw Lem, with his novel
Solaris and the Strugatsky brothers, with their novel
Roadside Picnic.
