In Naruto, there are many different rankings for missions. The higher the rank you are, the more dangerous and higher level the mission is. Below is a list of the different levels.

Rank S
These missions may not always be the most dangerous but only ANBU members or the Kage participate in them. It is very unlikely anyone of a lower rank would be part of these missions.

Rank A
This type of mission is the most dangerous. Ninjas ranked Jounin or higher are usually assigned to these missions. Very often, if things are very bad you will see a Kage participating. Generally you will not find a ninja of lower rank than Jounin in these types of missions unless it is absolutely necessary. Rank A missions are extremely dangerous and usually there is a death or severe injury as a result of it.

Rank B
Missions B ranked are not as tough as A ranked missions. The difficulty is still pretty high. Most B ranked missions involve retrieving important documents or protecting someone and taking them to a certain location. Mostly Jounin or Chuunin ranked ninjas perform these types of missions.

Rank C
These types of missions are generally performed by Chuunins. At times you will even see skilled Genins performing missions C ranked. Most C ranked missions involve protecting an elderly or sending/retrieving a document.

Rank D
D ranked missions are the lowest ranked. These missions are almost all performed by Genins. These missions usually are tedious work or basically useless work. Some missions include: saving an animal, taking out the garbage, walking an animal, or other pointless and boring work. Genins who want to be Chuunins must complete 8 of these missions.