All NPC corporations have agents and these are assigned to a corporation's individual divisions.
The type of mission an agent can give is determined by the agent's corporation and division.

The following table shows the chance that you will get a specific type of mission based on the agent division.

Division Courier Kill Mining Trade Accounting 90% 10%     Administration 50% 50%     Advisory 66% 34%     Archives 90% 5%   5% Astrosurveying 30% 40% 25% 5% Command 3% 97%     Distribution 95% 5%     Finance         Intelligence 15% 85%     Internal Security 5% 95%     Legal 50% 50%     Manufacturing 95% 5%     Marketing 95% 5%     Mining     10% 85% Personnel         Production 95% 5%     Public Relations 66% 34%     R&D 50%     50% Security 5% 90%   5% Storage 95% 5%     Surveillance 5% 95%

Courier missions
Ship one or more items from one station to another.
The agent level defines only the maximum cargo space needed and does not influence the minimum. (IE you can usually run any lvl1 courier mission on a frigate, while lvl3 missions can require from 0.1m3 to a full industrial ship cargo space)

Kill missions
Fight against one or more enemy ships.
The level of the agent influence the number and hardness of your enemies. Be careful when trying an higher level agent for the first times, you might be surprised to notice how harder each level is from the previous one.

Mining missions
Deliver a certain amount of ore or minerals to your agent.
The choice of buying or mining what you have to deliver is up to you.

Trade missions
Deliver a specific item to your agent.
Trade missions usually costs money since you have to buy what your agent is looking for (you can naturally sacrifice an item in one of your hangars if you already have it) but the cost is usually repaid by good agent rewards and higher standing gains.