This is the most significant difference between the game levels. The difficulty level affects how quickly the AI civilizations develop and expand. On the lowest difficulty levels, it takes the AI civs longer to train units, construct buildings and wonders, grow their cities and research technologies. As it was said, on Noble difficulty they play under the same conditions as the human players, and on higher difficulties they receive discounts on these items.
Moreover, on the higher difficulty levels, the AI begins receiving extra units to start the game. Notable jumps in difficulty come from the free worker that the AI civs start with on Monarch, the 2 free workers they get on Immortal, and the free settler they get on Deity - meaning the computer civilizations will have 2 cities from the very beginning!The AI civs also start with additional technologies on the higher difficulties.