Ogame designers have never really told, but through experience most people have found it works on a sort of probability.
You can expect Pirates (which will have levels lower than you) or aliens (with levels higher than you). Both of these are the combat scenarios and always unfavorable to have.
You can expect to return with nothing except some pets, return later than normal or earlier than normal.
Or the best luck of all, you can expect resources and ships.
This website
http://tools.ogamecentral.com/expedition-calculator#comment-928 can give you the basics.
I found this arrangement to give hundreds of k resources back with minimal battle encounters.
30-50 Heavy Fighters (They somehow carry a high rate to find good luck)
50-70 Large Cargoes
2-10 Battlecruisers or Destroyers
Espionage Probes
You can do expeditions in other systems other than your own, but with high deuterium cost. The chance of findings in a system is proportionate to the amount of people doing expeditions in that system. The less the better.