A Mark of Victory is a special award and is stored in your Personal Inventory similar to Pennants and Commendations.
PvP
Marks of Victory are rewarded for defeating other players in PvP combat. You only recieve marks now if the enemy player is 15 levels below or above you.
For each Mark Of Victory received, you also gain a Mark of Trade, exchangeable for different rewards.
A very few missions also reward Marks of Victory.
Trade
Marks of Victory are tradeable. This means they can be listed on the Auction House. This is to give PvP players a way to profit from their high-risk endeavors while PvE or Economy focussed players get access to the rewards provided by Marks of Victory.
Rewards
Marks of Victory can be turned in for various rewards at the Prize Agent in the Admiralty Office in your Nation's Capital for Nationals or the Crime Lord's Office in Tortuga for Pirates.
Note that you will only have the option to receive the highest level European Commendation that is available at your level.