Gay is a word used to describe someone who is homosexual. Today, the word usually means a person who is sexually attracted to people of the same gender. Often that person is also romantically interested in people of the same gender. A gay person can be a male who likes other males or a female who likes other females.
A person that is attracted to people only of the opposite gender is heterosexual. While a person attracted to people of the same gender and the opposite gender is bisexual.
The word "gay" can mean any homosexual person, but sometimes it is used just for homosexual men. In that case, homosexual women are called "lesbian" instead. Sometimes, people use the word "queer" to mean the same thing as "gay" when talking about sexuality.
In the early 20th century and before, "gay" used to mean 'happy' or 'carefree', but now people use it to define sexuality or to offend people.