Lesbian: A woman who is physically, romantically and/or emotionally attracted to other women.
Gay: A man who is physically, romantically and/or emotionally attracted to other men.
Bisexual: An individual who is physically, romantically and/or emotionally attracted to men and women.
Transgender: An inclusive term for people whose gender identity and/or expression differs from the sex they were assigned at birth. Transgender people may or may not decide to alter their bodies hormonally or surgically.
Transsexual: A person who experiences a mismatch between the sex they were assigned at birth and the sex they identify as being. A transsexual person sometimes undergoes medical treatment to change their physical sex to match their gender identity. Not all transsexual people can or desire to alter their bodies.
Queer: In the past, "queer" was a derogatory term, but now some LGBTQ people use it to describe themselves and their community. Others still find it offensive so it's best to use this word only if the person you are referring to has already identified as queer.
Questioning: People still in the process of exploring their sexual identity that are not ready to apply a label to themselves.