GoldenEye offers three different difficulty levels from the start, and a fourth after completing all of the game's missions on 00 Agent.
Agent is the game's lowest difficulty. Guards are weaker, less accurate, and Bond has fewer objectives to complete. There is usually additional body armor available on this difficulty. Auto-aim works well.
Secret Agent is the game's medium difficulty and usually has more objectives than Agent but less than 00 Agent. Secret Agent usually has more body armor than 00 Agent, but less than Agent. Less effective auto-aim than Agent.
00 Agent is the hardest of the game's three preset difficulties. Guards are at their strongest and Bond usually has more objectives to complete than on other difficulties. Body armor is usually scarce or nonexistent on this difficulty, and auto-aim almost non-existent.
007, often referred to as 007 Mode or (less commonly) 007 Agent, is a special difficulty unlocked after completing every mission on 00 Agent. It is something of a sandbox mode, allowing the player to set the health, damage, accuracy, and reaction times of enemies before starting a mission. It is generally designed to increase the re-playability of the game; applying the extreme high and low settings will far outstrip the difficulty presented by 00 Agent and Agent respectively, and thus 007 Mode can be used either for an extreme challenge or simply for fun.
007 uses the same objectives and object placement as 00 Agent, although the framework does exist to allow it to have its own.
It should be noted that the auto-aim function, almost non-existent in 00 Agent, works quite well in 007 mode.