Positive Feedback Of A Summing Amplifier
It's the result of the input (between + and - terminals) being amplified by the very large gain of the op-amp, and the finite supply voltage causing the output to saturate.  This is a typical "comparator with hysteresis".  Usually, it's best to use a device designed as a comparator to do this, not an op-amp.  Op-amps have compensation caps that can slow down the slew rate of the output, and some of them exhibit phase reversal or other strange effects in some comparator applications (though usually the strange behavior comes from violating the op-amp specs).  As others have pointed out the end result is not an amplifier, and the output won't change linearly with the input.