For women into women
Like all of the words people use to describe their identities, different people use the term 'lesbian' in different ways -but it's pretty much universal that lesbians identify as women, and have relationships only with other people who also do. Lesbians are, therefore 'monosexual', which means 'attracted to only one gender'. Not all women who are only attracted to women describe themselves as being lesbians, though - some prefer to use 'gay', 'queer' or other terms, for either personal or political reasons. It's always a good idea to ask someone how they self-identify before assuming, no matter how obvious you think they're supposed identity is.
What is a lesbian?
Some people who exclusively have sex with women and only find women attractive call themselves lesbian. Some chose other labels, depending on their preference.
How do lesbians have sex?
In a variety of ways, just like anyone and everyone else. It is a fallacy that a penis, or something penis shaped is needed for sex. There are many ways to have sexy fun without a phallus and lesbians most certainly know them all.
Do lesbians just have sex with women?
It will depend on the person, some may be attracted to just women, others may be attracted to femme presenting people. It is a personal preference and should always be respected. Sexuality is a very personal thing and is very much more than just the genitalia involved!
How do I know if I’m a lesbian?
Only you can really know. However, if you feel a romantic and/or a sexual attraction to other women you may well be a lesbian especially if it’s only women you are attracted to and no other sex. It is always up to you to decide what you label your sexual orientation as, though.