The diamond represents your Abstergo Rank. It's basically how the game compares you against all other players. In general, the more stripes you have, the higher your rank. And the amount of stripes is determined by what percentage of all players you fall under. If you are within the top 100, you will get a number instead. The Abstergo Ranks are reset at the end of every season. The season very recently ended on February 15, 2013.
The number to the side of the diamond is your level. The maximum level you can reach is 50. Once reaching maximum level, you will continue to gain levels again as if you started from zero again. For every subsequent time you reach your maximum level after that, you will prestige. White numbers represent levels a player is at. Yellow numbers with a star represents how many times the player prestiged.
In this screenshot, I
am was ranked 4th overall in the ladder and prestiged 9 times. In other words, I reached level 50, 10 times.