'Heterosexual' or 'straight' are words used to describe someone who is attracted to people of the opposite sex.
How do you know?
Within society it is assumed that most people are straight and so as you grow up you will probably assume that you will be straight as well.
Lots of people know from quite a young age whether they are attracted to boys or girls, but some people might not start thinking about relationships and who they fancy until they are older.
You can also be straight even if you have fancied someone who is the same sex as you. Being attracted to someone of the opposite sex is completely normal.
What is normal?
Some people think that being straight is 'normal' and that being gay, lesbian, or bisexual is ‘abnormal’.
This is not true; no sexuality is better or more normal than another.
However, more people are straight and people sometimes think that this means that being straight is better or morally correct. It does not, but it can mean that gay, lesbian, or bi people face prejudice and discrimination.
Besides, at the end of the day no-one is really 'normal' - whether you're gay, lesbian, bisexual, or heterosexual you're one of the remarkable individuals that make up the human race.
All the same
'Gay', 'lesbian', 'straight' and 'bisexual' are all just categories we use to describe who we are sexually attracted to.
In some ways these categories are useful as they help us to meet people who we can have relationships with, but in other ways they cause problems as they make us think we are very different when we are not.
It does not matter if you are gay, lesbian, straight or bi - when it comes to having sex, forming relationships and falling in love, it is the same for everyone.