At first they had very different personalities, but both girls became friends during their teens years and then eventually best friends.
As the series begins, not only are Brooke and Haley from differant groups of friends, they are also completely different. Brooke, being the captain of the cheerleaders and most popular girl in school didn't take much of an interest into Haley, who was an 'outsider' and a math tutor, and Haley didn't really like Brooke to begin with. When they first met, Brooke had twisted her ankle and was given medication to take, but ignored the doctor and took them all at once, leaving Brooke light headed and giggly. Brooke names Haley, Brooke and invites her to go back to Tree Hill with Peyton and herself.
Brooke gets married to Julian. Peyton couldn't be maid of honor because Sawyer was ill, so Haley takes her place. During her speech she says, "Well, when I first met Brooke Davis she had no idea who I was. Of course, everyone knew who she was. She was the most popular girl in school. She was head cheerleader, annoyingly pretty. Still is. And I used to wonder back then, "what it would be like to be friends with Brooke? Would she make me popular? Would all the boys start asking me out?" And then one night, I got to hang out with Brooke all night. And, oh, she didn't like my name, though, so she decided she was gonna give me a name that she did like, which ended up being Brooke. And from that moment, I knew that I wanted Brooke to be my friend, not because she was gonna make me popular and not because boys would start asking me out, but because I got a glimpse of the real Brooke, a girl with the biggest heart that I have ever known. And, you know, when you're in high school, it's not very easy to let people see who you really are. I could see it, though. It was in the eyes. So we became friends, and now she's my best friend. We've been through so much together over the years. And our friendship is still growing. So I know that your friendship and love for Julian will continue to grow every year for the rest of your lives. And I feel so lucky to be a part of that. I love you, Brooke Davis."