I am an INFP
Type of my best friend:
ISTJ
My friendship with her
We tell each other everything. She is one of the few that genuinely got acquainted with the layers of my soul...and vice versa. She listens to all my nonsense (crazily vain and cold-blooded streak) and made-up for entertainement conspiracy theories with a raised eyebrow and in return, I am there to calm her down when she's in her panick-mode(does that a lot!). We bicker for fun, have those amazing debates on life twice a month--we have opposite views on everything but that's what makes our friendship so great in the first place. Although we respect each other, we don't hesitate in delivering brutal honesty --we don't want to spoil our bond by walking on eggshells all the time. As soon as she is upset, I take the car to pick her up for a ride& some starbucks, and when I am in one of my moods, she serves common sense without taxes

We rarely fight.
Since we go to the same university and are both in our second years, I see her almost every day at dinners(our group of friends eat at the same faculty each day at the same time). And on the week-ends, we study together at the library. She is in admin to become an accountant, and I am an english major, so we don't have any classes in common, but we still prefer to study together(helps with my motivation anyway) and talk in our breaks. We go out every two months to the movies, to do sleepovers or at a restaurant, half the time joined by other friends of ours too.
We don't communicate online because I don't use my msn and facebook account much--8 times a year-but all of my friends have learned to not count on me chatting online because I prefer to use my alone time otherwise.
As for boundaries..well, not really. We know what buttons not to push(teasing about touchy matters, mostly) but other than that, we manage to be honest with each other without offending the other. I am rarely offended anyway, and I know how to use my diplomatic skills to avoid her spitting fire all over me. I don't beleive in shugarcoating anything so I don't, but apparently something in my voice(gentle firmness or whatever) keeps people from getting too much offended in my presence. Maybe because I look like a lame angry turtle when I scold...which is unsurpisingly isn't scary and doesn't invite a lot of reactivity. *Shruggs*
What made us best friends:
Well, we first bonded over the fact that we weren't shallow like most people in our school. We thought getting drunk at parties wasn't fun at all, and thought there was other things to do with our lives than sit on our arses and mock people(which is the favorite hobby of many girls, apparently).We liked to talk rather than play some sport, so we began confiding into each other pretty soon. We have both a fascination over tarot cards, dreams, the unconscious, so we had interesting conversations on those topics. Although she rarely bothers to think about philosophical stuff(thinks it's interesting but has other priorities), she likes to debate and win(and honestly..likewise

..), so I bring up philosophical topics out of the air, and we debate every aspect

We also have both a strong moral side, so questions of injustice have us ranting at the planet for hours.
So yeah, a few commonalities to start, and then, the fun of exploring our differences through debates. we just get along with each other really well. Natural compability, I would say.