An Encounter mission is a mission to go to a location somewhere in space and complete an objective of some kind. The objective is usually to kill a ship or a set of ships located at the encounter, but it could also be to destroy a structure, to get close to a location and then escape, to pick up an object at the location (which may or may not be an ambush), or to fly from beacon-to-beacon.
An Encounter mission will always create a mission space when the mission is accepted. A mission space is a region of space in some solar system and is populated by objects also created specifically for the mission.
Mission spaces will often contain acceleration gates to move around the deadspace with; these are often locked until nearby enemies have been defeated.
Encounter missions can usually be recognized as such when reading the description by looking for a bookmark link that only has a solar system name. For example, if a mission description has a bookmark link that only says "Aldrat", then it's definitely either an Encounter or Mining mission.
A Mining mission is just like an Encounter mission, except that the mission objective is mining-oriented. This excludes certain Encounter missions that require a mining laser as a gimmick, where you bring one mining laser to a mission space to mine an asteroid for the purpose of luring in a target ship and then destroying the target ship. Such gimmick encounters don't care about how much ore you mine, and any ore you mine is completely secondary to the mission.
Mining missions, on the other hand, require you to mine an asteroid or set of asteroids in a mission space until the asteroids are depleted and bring the ore back to the agent's station.
There is a risk of combat in mining missions, though the hostiles that show up tend to be much weaker than hostiles found in encounter missions. It is advisable to have some offensive capability (like a set of combat drones) or have a strong enough tank that you can basically ignore any hostiles that show up and start shooting at you.
The mission may require you to mine more ore than can fit in your cargohold; this is typical of mining missions. Level 1 missions will require mining up to 2000 m3 of ore, level 2 up to 6000 m3 of ore, level 3 up to 9000 m3 of ore or 10000 m3 of ice, and level 4 up to 45000 m3 of ore or 20000 m3 of ice.
A Courier mission is a mission to take a piece of cargo from one station to another station. When a Courier mission is accepted, the necessary cargo is spawned in your personal hangar at the pickup station. You then need to haul it to the destination ("drop off") station using your ship. Once docked at the destination station you may complete the mission by talking to the agent. The cargo only counts as delivered if it is either in your personal hangar at the destination station or in your ship's cargo hold while you are docked at the destination station.
Courier missions never spawn any hazards of their own; you only have to deal with the normal hazards of Stargate travel (gatecamps, suicide gankers, warp interdiction bubbles on Stargates in NullSec, the sovereign space of Empires that hate you, and so on). Level 1 missions will keep you within the agent's constellation, level 2 and level 3 will possibly send you to a neighboring constellation, and level 4 courier missions will always send you to a neighboring constellation.
It is worth noting that although you are at the destination station you can still talk to the original agent remotely to complete the mission without going back to the original station. They will be listed in the station's "Agents" tab after you dock, or you can start a conversation with them using your mission journal. However, until you fly back and dock at their station, they will not give you a new mission.
If a Courier mission has an item as a reward instead of ISK, then the item will appear in your personal hangar at the agent's station (which may or may not be the dropoff location for the mission).
Level 1 and 2 courier missions can be run using frigates, although you may need to use cargo modules in the low slots. Cargo size for L1/L2 missions can be up to 450 m3 in size. For level 3 and 4 courier missions, you will need an industrial hauler because cargo sizes will be in the 4000–8000 m3 range.
A Trade mission is to obtain a quantity of some material and deliver it to a destination station. Unlike a courier mission, the necessary materials are not spawned for you, you have to obtain them yourself: mine it out of asteroids, buy it off the Market, steal it from another player, and so on. How you get the materials is irrelevant to the mission.
A Deadspace mission is to unsolicited entry in another player's mission deadspace area. These type of missions Invading deadspaces is generally done to hunt down mission runners (PVP), but is also done to ninja loot and/or ninja salvage.
It's recommended to use a ship that grants probe signal strength bonuses. There are also some implants that grant similar, related bonuses. This may help cut down the time you need to track down a mission deadspace.
You need to locate a ship inside a mission deadspace by using probes. Send combat probes in empty areas of space with a fairly large scan radius. If the probes pick up a ship in the middle of nowhere, chances are it is located in a mission deadspace.
All that needs to be done is getting 100% signal strength on the ship and warp to it. You will be warped at the entrance of the deadspace, at the acceleration gate. You should make a bookmark of the entrance.
It is believed that mission deadspaces last until the mission runner completes the mission. However, if you are still in the mission deadspace, it will not disappear until you leave (or any other players in the deadspace). They also disappear during downtimes.
After every 16 regular missions completed you will be offered a storyline mission. These will be one of the regular mission types but differ by having a significant impact on your faction and corporation standings. The initial 16 missions can be completed for any agent, as long as those agents are aligned with the same faction and are at the same level as each other. The number of missions is not altered or reset by rejecting or failing an accepted mission.
Most of the Cosmos missions may only be done once, but they provide unique rewards and give a significant boost in standings. However, pilots have to find Cosmos agents inside complexes to receive such an offer.
Epic mission arcs are composed of a series of branching missions that present the player with choices. These choices will determine how the Epic Arc concludes. These missions also increase your faction standing considerably.