Alien UFO's swoop in regularly and take over territory on the World Map. UFO Landing Sites have clearly noted levels on them that tell you how difficult attacking them will be.
You can also simply tap on the landing site and tap Report to see exactly what alien troops are stationed there. Unlike scouting another player, Report does not alert the aliens to your interest.
If you can defeat all the stationed alien troops, you'll be rewarded with a variety of rewards including alien technology! Alien Components include items like Alien Power Cores, Alien Tech, Alien Armor, and much more. These are required items when performing high-level building upgrades and research!