Unisex shirts are regular t-shirts that have no "form" that would specify gender-specificity. Like what you'd get at a rock concert:

http://is.gd/35gkQ

Basically, you shift the sizes down by 1 increment (Medium for a man would be Large for a woman, etc.). This is pretty common and girls looks just fine in unisex shirts.

Tim

P.S. I wish more girls would attend the Summit as well.