Since the start of the equal rights movement, the acronym and identifiers have changed and expanded to encompass a broader range of identities. As more studies into gender identity and sexual orientation are expanded and undertaken, the definitions are likely to keep adapting to fit new information. As of now, the initialism has expanded to include Lesbian, Gay, Bisexual, Transgender, Queer/Questioning, Intersex, and Asexual.
Some of these terms are umbrella terms that encompass a larger range of more specific identities. For example non-binary gender, gender fluidity, gender androgyny, and agender are specific gender identities that are included under the transgender umbrella; aromantacism and demisexuality are included under the asexuality umbrella.
It is a constantly shifting spectrum of identity that grows more comprehensive, as more is learned about human self-identification. For a list of the basic definitions for the terms included in LGBTQIA, check out some comprehensive blog entries and support websites: here and here.