Occupation

In Rome - Total War when you conquer a settlement after a siege or battle you have the option to Occupy, Enslave or Exterminate the population.

In Barbarian Invasion if playing as a horde race that has not yet settled you have the option to either occupy or sack the city and move on.

Occupy

Gives a small amount of money and leaves population intact.

Enslave

Disperses population amongst other governed cities.

Exterminate

Gives larger amount of money and substantially reduces population.

Sack

Only available to horde nation, gives large amount of money, reduces population and damages or destroys most buildings.