Alien Missions

Research

Generally small vehicles, least threat

Harvest

Great concern to governments, usually cattle abduction with Harvester UFO.

Abduction

Abducts humans, causes great alarm.

Infiltration

Infiltrates countries and try to make pact with government, big threat as countries that sign pact cease to fund XCOM!

Four UFOs take part in an Infiltration mission, landing in both the target and neighboring countries. Limited information indicates two of these are often Battleships which must be shot down to forestall the infiltration. Engaging intact UFOs on the ground is apparently not enough to prevent them from completing their mission.

Zombie says "According to my preliminary results, Russia cannot be infiltrated in the CE version of X-COM so losing the game by this method is impossible." (Jun 16, 2005, StrategyCore forums)

Base

Survey and establish alien base, may contain alien commander, esp. higher levels

This commander does not yet count as a Base Commander so is not useful for 'research' purposes.

Terror

Creates terror sites when lands at city, so get them fast!

Contains Terror Units. Failure to intercept will result in a Terror Mission.

Retaliation

Scout / attack XCOM base, greatest threat

Scouts can be misled. If you shoot them down further and further away from your base each time, their search pattern will never find your base and they may eventually give up. If they do find your base, they'll send battleships.

Shooting down an incoming battleship will only delay the inevitable Base Defense mission, as another one will soon be on the way. Best prepare the base.

Supply

Supply alien bases. Trail them and patrol nearby to find alien

The steady stream of supply ships to an alien base can mean several fairly easy supply ship missions for the capitalist XCOM commander with a well-placed radar post.

(Outline from section 9.4 of KC-USG , available in Links)