The main focus of players of Elite Dangerous is to achieve the coveted Elite rank in different disciplines. While the exact calculations are not known. The earning of the security, trading and exploration ranks is based upon how many credits you have earned in performing tasks.
|Security Ranks||Trading Ranks||Exploration Ranks|
|Mostly Harmless||Mostly Penniless||Mostly Aimless|
This means that taking high value missions for assassination of a single target would be a quicker way to earn your security rank than many low value targets.
With factions, ranks are a bit different and you are currently provided a rank after you have completed a number of missions. I would say the current faction progression method is most likely due for review and change as I quickly sprinted through several Imperial ranks with very little work being done. That was after I became friendly with them while working as a mercenary during the Sorbago rebellion.
|Federal Rank||Imperial Rank|
|Chief Petty Officer||Knight|
With some faction ships, players will need to earn a certain rank before they are able to purchase them. Currently that is just for the Imperial Clipper (Baron) and Federal Dropship (Ensign), but I assume more will be added at a later date.