Some may identify their gender as right in the middle between male and female, others may feel mainly male but not 100% male (or vice-versa not feel 100% female), and yet others reject having to define their gender in terms of male and female in any way.
The idea has grown from the rejection of the strict binary split of gender which makes some people feel they are pressurised to conform to certain types of male or female behaviour to be accepted.
No fixed definition
There is no firm definition of exactly what is meant by third-gender or gender-queer and most people who use either of the terms to identify themselves have no particular wish for the constraints which a clear definition might bring.
People who call themselves third-gender or gender-queer also span a very wide range of desire to transition. Some have no interest at all in physically changing their body, some wish to partially physically transition (for example taking hormones but not having any surgery) and yet others follow the same transition route as transsexual people do but reject identifying simply as the gender they transition to.