Human Biology help: what's the difference between a positive and a negative feedback?

Negative feedback, when the production of something slows down it's own continued production.

Example: TSH and FT4.  TSH stimulates FT4 production.  When there is enough FT4, increased concentrations of FT4 send a signal to stop making TSH.  If you didn't have this, TSH just keep stimulating more and more FT4 until there was too much.

Pos. Feedback, production of something sends signal for to increase it's own production.

Example: Contractions in childbirth.  Contractions produce oxytocin, which sends signals to increase contractions, in order to give birth the the baby.  Also, blood clotting.

hope that helps somewhat.

Positive feedback.  An action causes the production of a compound which in turn causes more of that action to occur.  An example of this occurs in childbirth.  When a contraction occurs, the hormone oxytocin is released into the body, which stimulates further contractions and so on. This results in contractions increasing in amplitude and frequency.

Negative feedback.  An action causes the production of a compound which in turn stops the action from occurring further.  This is very common in hormone function.  For hormone secretion regulated by the negative feedback loop: when gland X releases hormone X, this stimulates target cells to release hormone Y. When there is an excess of hormone Y, gland X "senses" this and inhibits its release of hormone X.