"Gender" means the quality of being a male or female. For example:
They split us up by gender.
Everyone, regardless of gender, should have an equal opportunity to get an education.
Another word for "gender" is "sex". For example, you can ask a pregnant woman:
A: Do you know the sex of the child?
B: It's a boy.
In recent years in the U.S., there has started to be a small difference between "sex" and "gender". You can read about the difference here: http://www.med.monash.edu.au/gendermed/sexandgender.html