You can earn ranks by winning contests or having lots of forum posts. Fifteen posts is 1 BP. The ranks are called Bloons Points, or BP. Here are the ranks for BP.

0 BP- Human
1 BP- Bronze Monkey
3 BP- Platinum Monkey
4 BP- Gold Monkey
6 BP- Bronze Chimp
7 BP- Platinum Chimp
9 BP- Gold Chimp
11 BP- Bronze Ape
13 BP- Platinum Ape
15 BP- Gold Ape
18 BP-Bronze Captain   
20 BP- Platinum Captain     
23 BP- Gold Captain
26 BP- Bronze Lieutenant
30 BP- Platinum Lieutenant
34 BP- Gold Lieutenant
37 BP- Bronze Prince/Princess
40 BP-  Platinum Prince/Princess
45 BP- Gold Prince/Princess
48 BP- Bronze King/Queen
51 BP- Platinum King/Queen
55 BP- Gold King/Queen
59 BP- Bronze Super Monkey
64 BP- Platinum Super Monkey
70 BP- Gold Super Monkey