Transgender is a term used by some people whose gender does not match the one they were assigned at birth. Its opposite is "cisgender". Being transgender is an identity which can intersect with gender and other identities. For instance, trans women experience forms of oppression that cis women typically don't experience, as well as experiencing most forms of oppression that cis women in the same culture who have similar intersecting identities do experience.
Not everyone whose gender does not match the one they were assigned at birth uses the term "transgender" to refer to themselves; some people describe themselves as "transsexual" or simply "trans". Avoid describing an individual as "transgender" or "transsexual" unless you know which labels they use to describe themselves. Alternatives for referring to groups of people include the adjectives "trans", "gender non-conforming", or "gender-variant"; or the noun phrase "members of gender, romantic and sexual minorities".