WTF are ranks?
It's a direct representation of your Honor Ranking. When you PvP, you get Honor. You get it from killing a player within 10 levels of the opposing faction, you get it for turning in flags in WSG, you get it for resource captures in AB, etc. The more honor you get, the higher chance you have of beating more people on your same server/faction.
For Alliance, the ranks are:
1) Private
2) Corporal
3) Sergeant
4) Master Sergeant
5) Sergeant Major
6) Knight
7) Knight-Lieutenant
8) Knight-Captain
9) Knight-Champion
10) Lieutenant Commander
11) Commander
12) Marshal
13) Field Marshal
14) Grand Marshal
Getting rank 14 requires immense amounts of time. You can pseudo-lightly PvP to get to 13 (meaning you play for 10-15 hours a week, possibly more as your progress), but when you get to 13 and want to increment your rank to 14, your honor ranking has to be
#1 for
three weeks in a row. To clarify, that means you have to PvP more than anyone in your server + faction for three weeks straight. That's about 10 hours of PvP a day for 21 days. It's a ridiculous amount of work for Weapons (you can get the full armor set at rank 13) that aren't as good as weapons that drop in instances. People do it for the prestige, not the gear.