Capacity means how many soldiers can be carried (1 vehicle is equal to 3 soldiers). Speed is in kilometers per hour. Geoscape distances are the same as on the Earth. Range means how far between two POIs aka sites can aircraft travel. Durability is in Hit Points.
Tiamat has the highest capacity and range (range can be increased further with Advanced Tiamat Development research) but lowest speed. Helios has the highest speed but smallest capacity. Thunderbird has the highest durability.
Aircrafts can be repaired in Vehicle Bay facility. Manufacturing Aircraft can be fastened up by building more Fabrication plants. You can also steal aircraft from Factions. Why should you steal Helios. Steal aircraft mission type explained here.