I haven't played the game, so I can't tell you the difference between the trail version and the paid version, but here are the features of the game:
-Skies of Glory is a 3D aerial combat game set in the skies of WWII.
-Stunning graphics: multiple terrains including mountains, deserts, and sea to showcase the authentic views of WWII air combat.
-Engage with opponents from around the world in 8-player combat over the internet via Edge, 3G and WiFi
-Fly with friends over any local network for up to 8-player combat
-10 Campaign Missions to complete in the Pacific and European theatres of war
-Multiple skirmish modes and and tons of training missions with more to come.
-100 Dogfight Training exercises.
-Access planes and additional missions via in-app purchase
-Regular updates ensure that you will never run out of new missions or planes
I'm guessing the trial version is limited to the amount of planes you can fly, missions you can complete and game modes you can play. Since you tried it, you should be able to compare what you were able to do with what features are included in that list.