What types of signals is SETI (and other such efforts) looking for? This seemed to be the closest group for this question.

Every few months there are a few articles asking "Are we alone in the universe" which focus on the Fermi Paradox, the Drake Equation, and end by talking about SETI and its lack of progress. As I understand it SETI is looking for signs of technological civilizations beyond our solar system, both in our galaxy and beyond through radio astronomy. 

My question is about what would count as a "sign" of a technological civilization, and is there a maximum distance beyond which such a signal would be indistinguishable from other radio emissions from stars and galaxies?
What signals do SETI receive?
SETI uses radio telescopes to look at large portions of the sky across a range of wavelengths. They believe that we are most likely to find alien civilisation buy looking for microwaves.

How would we know if they were from aliens?
There are radio signals from many different sources bouncing around the universe, many of which we don't quite understand yet. However, natural sources of energy tend to produce a range of wavelengths. SETI are hoping to find "narrow-band signals", which  are strong signals at a single wavelength. These (as far as we know) only come from devices made to produce them, so only a technologically advanced species would emit these signals.

Is there a maximum distance for detecting these?
Yes, these signals would decrease with distance. It is not so much that they become indistinguishable form natural sources, as they are a different type of signal, but as the waves travel they spread out, like dropping a pebble in a pond, so the signal is much weaker as you get further away. Also, space is not empty. If the waves are emitted from a planet in a galaxy, there will probably be lots of dust and gas (interstellar medium) in the galaxy which will absorb the radio waves, so it might be like looking for car headlights on a foggy day - we will see something vague and fuzzy but it will be much weaker if there is more matter between us and them.

If you want to find out more about SETI they have a website with FAQs and news. If you want to get involved there is a project called SETI@home which uses spare processing power on your home computer to analyse the data they are receiving and search for these narrow-band signals.