We have no idea what life on other planets, if it exists at all, looks like. We can't tell how similar or how alien the aliens are. Hence, a science fiction author has quite much leeway in this respect. However, most scientists assume that they are much more alien than we can imagine. Life has taken many forms on Earth, and on a different planet with perhaps vastly different environmental conditions, it might have taken any of the infinitely many paths life could have taken on Earth but didn't -- or perhaps one that life couldn't have taken under Earth conditions.

Of course, there are natural laws which hold everywhere in the universe. Living organisms use up energy, and therefore need some energy source. Sunlight, as on Earth, is an obvious choice, especially for life on the surface of an Earth-like planet. However, intelligent beings probably don't harvest solar radiation directly, but consume organic matter built up by other organisms. This is because this way, they can access the solar energy of an area much larger than their own skin. This is why life on Earth is divided into the realms of plants (sessile, light-harvesting organisms) and animals (mobile organisms feeding on other organisms' organics).

One aspect which is often insufficiently considered in designing alien intelligences is alien psychology. Why should beings of an entirely alien design, living in an alien environment, think and act like humans? It is much more likely that they have concepts and motivations entirely incomprehensible to us.

Some of these differences are not that hard to understand at all, as they are consequences of different environment and sensual organs. Imagine a dialogue between a human and an alien (I admit this is a silly example, but I think it marks out the point):

Alien Here's my e-mail address. (Passes a slip of plastic.)

Human Where? I can't read anything! That card is just plain black!

Alien Oh sorry, I forgot that your primitive eyes are insensitive to infrared... by the way, why are Terran Navy uniforms black?

Human One point for me: our Navy uniforms are not black, they are blue!

However, there might be differences which are far less intelligible than that. The aliens might do things which, to us, just don't make any sense. To them, it might be entirely logical! Or they might display patterns of behaviour we would abhor, and vice versa. (`What? You human females don't eat your males after mating? What a disgusting habit! And I can't imagine that anyone in power can be so stupid to ask her subjects every four years whom they want to govern them next, and if the subjects want another government, just resign, instead of killing and eating her opponents?')

Even more alien in their thought patterns would be aliens which live in environments entirely different from ours. A being from an Earth-like planet might (or might not) be able to appreciate the beauty of a clear summer saturday afternoon on Earth, but a being from a Venus-like world certainly couldn't -- just as we cannot appreciate what to them is good weather; and on the other hand, for a being coming from an icy liquid-methane environment like Titan, our Earth is just a searingly hot place where much of the rocks is molten.

Some alien races might be so highly developed that they don't even consider humans intelligent. They might be thousands of millions of years ahead of us, which means that they are as advanced over us as we are advanced over bacteria. We cannot say how such beings react to us. They probably ignore us -- but they might ignore us so utterly that they don't care about our existance! They could scramble down onto Earth to dismantle it (because they need building material for some vast, incomprehensible mega-engineering feats), not caring at all about what lives there.