Your titular query would leave room for misinterpretation, were it not for the remainder of your question,
What is the word or phrase that describes a person's teeth whose bottom teeth protrude over their upper teeth?
The "word or phrase" most familiar to me, describing that phenomenon, is
undershot, adj. (and n.)
2. Having the lower jaw or teeth projecting beyond the upper; underhung.
["ˈundershot, adj. (and n.)". OED Online. December 2015. Oxford University Press. http://www.oed.com/view/Entry/212031?redirectedFrom=undershot (accessed March 01, 2016).]
'Undershot' may or may not be more precisely descriptive, depending on the context, if it is used as an adjective along with the relevant defining noun, as in
undershot tooth or undershot teeth
You asked about people, so I'll limit myself to a couple of images of people with undershot teeth produced by an online search (Ecosia):
The condition, however, seems to be much more common among dogs and rodents.
While 'underhung', as suggested by the OED Online definition of 'undershot', might seem at first blush to be a more or less exact synonym of 'undershot', it is only an adjective (disregarding colloquial and disparaging uses related to other parts of the anatomy). Further, the definition of 'underhung' suggests a condition of the jaw, rather than a condition of both the jaw and the teeth (etc.):
a. Having the lower jaw projecting beyond the upper, or coming unusually far forward.
b. Projecting beyond the upper jaw.
There appears to be controversy regarding 'buck teeth' (also 'bucktooth' or 'bucktoothed'). Various lexicographers admit a shade or two of disagreement: American Heritage and Collins specify that 'bucktooth' applies only to upper front teeth; Random House Kernerman Webster's uses especially to relate 'bucktooth' to upper teeth, while not ruling out projecting lower teeth; OED Online doesn't weigh in to favor either upper or lower.