What is a blackhead? Can someone show me a link to a picture?

http://www.acne.org/types-of-acne.htmlWhat is a blackhead? Can someone show me a link to a picture?
A bird disease


http://www.michigan.gov/dnr/0,1607,7-153鈥?/a>





acne like on humans


http://adam.about.com/encyclopedia/Black鈥?/a>


http://health-pictures.com/acne/blackhea鈥?/a>