Homosexual is a noun and adjective. Either way, it has to do with people who are attracted to members of the same sex.
Homosexuals are attracted to people of their own sex in romantic and sexual ways. A lesbian is specifically a homosexual woman, and the word gay applies to men and women who are homosexual, though more often men. Homosexual people have faced a lot of prejudice, just like women, African-Americans, and other groups. But society is working to support homosexual people as equal citizens with equal rights.