This is certainly the best case scenario, no immediate need to rush weapons in to defend a colony of miners or whatever, so there's no need for alarm on both sides, a gradual understanding, then co-operation builds up. As I said, 'Best Case' scenario, no-one to get hurt but a few robots and someone's pride. Another similar scenario is they find us, we're still advanced, say an alien robot ship rocks up to a system where a small Human mining crew is working. We do what we do best, and panic. "Fire Phasers!" would be the quote of the day. The alien's reaction varies greatly, an "Oh My God, they just destroyed our robot, FIRE PHASERS!" reaction seems likely. A more ethereal race might have the reaction, "Oh...they destroyed our robot, oh well," and move onto the next system. While a civilization like our own, would panic, then wonder if they got it right, and send more robots just to confirm we were destroying them. Hopefully, interstellar leaders would not be inept, and pull off a good old apology, explaining panicking, xenophobic miners were to blame, and look forward to interstellar peace...blah, blah, blah. But if the reverse occurred, and we found a bunch of their miners, chances are similar, they'd panic and start firing phasers, in their defence, chances are we were poking them a bit to hard, we get a bit too excited over things like that. And our reaction, as stated would be to send more robots to make sure they were actually destroying them and we hadn't crashed into a phaser-shaped meteor.
What happens if we find them? What happens if we come across a less advanced race? Well looks we're gonna be the ones abducting a few to do tests, which probably won't do wonders for interstellar diplomacy. It depends on how advanced they are, how unified and the social state of the planet. If we meet a unified planet, with a global government, it seems likely we'll play the benevolent Gods, chances are they might have something we want, and we'll trade some of our older technologies for whatever they have. If the planet is in a less advanced state, such as our medieval or turn of the century societies, our reactions could be much more varied. We would obviously study them, being superstitious and lacking any global communications (assuming they're non-telepathic or similar physiology) or global defences, we pretty much have free rein to do whatever the hell we want, which is never good for anyone, look at examples on Earth, whenever a Colonial civilization encountered an indigenous civilization, it resulted in cultural loss and in some cases genocide.
A similar situation might occur here, with some much free rein, it would be relatively easy for groups with political or ideological objectives to establish a foothold, promote their particular beliefs, and arm their followers with modern weaponry, turning the planet into a battle of ideologies and beliefs. Which is why whichever government is in place must take action to secure and seal off this untouched planet, so research, as non-invasive as possible, can be undertaken. Even this little unadvanced planet and people might have something to offer us, breeding at 100 times our own rate, we could use them as a cheap labour force or as an army. Being worshipped as Gods, if we were to come into contact with them in very early advancement, could be beneficial to both species, as it allows us a greater authority over them (assuming we not being tools about it), while themselves also benefiting from the "Gifts of the Gods" eg: farming, irrigation, medicine.
But, for the million dollar answer, what would they look like, and how would they behave? If we assume they're intelligent and not just standard fauna (ie: cows, sheep, birds), we can actually make a fairly good guess at what they might look like, and to some extent how they might behave. First and foremost, limbs. They will have limbs that can manipulate their environment in an advanced way, (from here on in, when I say 'limbs', I am referring to these manipulative appendages, not just legs or arms). We have hands with opposable thumbs, for example. They may have pincers or other such appendages that we are unfamiliar with. They're senses will also be reasonably advanced, sight is the most likely, although very sensitive sonar could also be likely, depending on the environment they live in. A third possibility, that has other knock on consequences, and its likelihood is debatable, is the possibility of a singular intelligence that aggressively infects and then controls various species which may not be intelligent, like a virus or parasite that has a consciousness. The senses it uses cannot be guessed.
