Positive feedback refers to a process that respond with increasing intensity to the stimulus perceived. A good examply is oxytocin release in child birth. When the mother feels the contractions, oxytocin is released. As contractions get more painful, more oxytocin is released. This helps to keep continuity in the process and ultimately the baby is born.
A negative feedback refers to a process that tends to regulate the initial response deployed. For example, blood sugar levels regulation. When we eat, as food is digested it raises blood sugar levels. This triggers insulin released by the pancreas. Then, sugar is ingested by cells so blood sugar levels go down. This sends the signal to stop insulin release.
So, to sum up, a negative feedback is a process that responds towards diminishing the stimulus effect, while a positive feedback intensifies the physiological response towards the stimulus.