Alien Missions (Long War)
For missions involving multiple UFOs, usually the last UFO needs to be defeated to foil the mission. Larger ship types become available for a mission only if the aliens have enough resources, and the final ship chosen will be a random selection of those available. For example, if the aliens are weak then they will randomly send either a Scout or Raider for research, if they are strong then they could randomly send a Scout, Raider, Abductor or Terror Ship for the same mission.
If the aliens send a UFO and it is damaged (but escapes), the aliens will pay resources to repair it, limiting their options in the next month.
The aliens have a strategy in Long War, and tend to focus on geographic locations. Expect to fight them the most in the continent where they lay their first base. 
A Scout, Raider, Abductor or Terror Ship will land for a while and then fly off. This mission will grant the aliens research if successful. Expect dozens of high-level aliens, even packed into the tiny Scout, especially if you allowed it to land for maximum artifact retrieval.
A Scout, Raider, Fighter or Destroyer will fly around the target area. Successful scouting missions are precursors to satellite hunter launches.
A scouts base chance of success on finding a satellite is 60%, 90% on the second pass. (With the Stealth Satellite upgrade, this drops to 30/50%). Damaging the scout will decrease its chances, 50% health being an automatic failure.
A Transport, Harvester, Raider or Scout will land for a while and then fly off. This mission will grant the aliens resources if successful. If the aliens are sufficiently "encouraged", they will pay a small resource fee to pick a harvest location where XCOM does not possess a satellite.
A Fighter, Destroyer or Battleship will fly around the target area hunting satellites. This mission will destroy a satellite if successful. Damaging the UFO will reduce its chance of destroying a satellite at the end of its mission.
A Fighter, Destroyer or Battleship will fly around the target area bombing civilians. This mission will cause a lot of panic if successful. Damaging the UFO will reduce the amount of panic created at the end of its mission.
An unfettered air raid generates 8 panic in the country raided, in addition to the 2 panic added from ignoring the UFO.
An Abductor will fly to the target area and generate an Abduction site. A successful abduction will increase panic in the country by 25, and panic across the continent by 20 (in all difficulties but normal). Thwarting the abduction will decrease panic in the country by 2.
A Terror Ship or Assault Carrier will fly to the target area and generate a Terror site. The aliens will terrorize the most panicked nation roughly 75% of the time. There is a limit of 2 terror missions per month, but they can be stacked at the end and beginning - unlucky commanders who have ceded many resources to the aliens may see as many as four in a short period of time. Terror missions are resource-intensive, and keeping the aliens on their toes should limit their ability to generate them.
Terror missions increase panic by +2 in the targeted country/+1 continent-wide per civilian killed, so these missions are always likely to see some benefit for the aliens. Losing or ignoring the mission will cause the immediate construction of an Alien Base in the country, and increase panic continent-wide by 20.
If the Terror Ship or Assault Carrier is badly damaged by XCOM interception attempts, it may abort its mission and return to base - though it's not easy without good craft and weapons. If forced to withdraw, the UFO will sharply change direction (generally doubling back towards its point of origin) and accelerate to maximum speed. Whilst further interception attempts are unlikely to catch up with it (making UFOs on terror missions some of the hardest to actually shoot down), simply avoiding a potentially costly terror mission can be worth the initial risk.
In late game up to 4 ships with Terrorize Populace mission via Hyperwave decoder may be seen. For each planned Terror site, first the Terror Ship will fly over targeted country. After several days, Assault Carrier will arrive. The Terror Ship appearance may be useful information where to relocate Firestorms. Shooting down Terror Ship does not prevent Assault Carrier arrival.
An Assault Carrier (or possibly an Overseer UFO) will fly to your main base or to one of your interceptor bases and immediately start an XCOM Base Defense mission or an Air Base Defense mission . The aliens can only field this mission if they have a lot of resources, and they will only do it if they consider XCOM a high threat (assaulting one of their bases will usually convince them of this, though sufficiently inconveniencing them before then will also do it).
Aliens usually send an Assault Carrier to main base assaults. If the aliens win a base defense it will damage resources and kill personnel - but it is not the end of the game, and can be recovered from. XCOM will lose the following resources on a failure:
- 450-550 cash
- 50-60 alloys
- 50-60 elerium
- 100-110 meld
- 20-30 scientists (capped to 1/3 of current staff)
- 20-30 engineers (capped to 1/3 of current staff)
- All alien captives
- Interceptor damage
For Air Base assaults, aliens will send an Overseer. The cost of failure of these missions is the destruction and/or severe damage of all stationed interceptors.
A Battleship and Transport will fly around the target area, followed by an Overseer inserting in to the target city. This mission will cause the target country to leave XCOM immediately if successful. This mission requires a huge amount of resources from the aliens. Note that the final Overseer can only be detected if XCOM has the Hyperwave Decoder, so without one the country will appear to leave for no reason.
An Overseer will fly around the target area, giving XCOM an opportunity to shoot down an Overseer.