Clarification on terms associates with end of training

I've been tweaking and trying several different training algorithm, activation function, number of hidden layer, number of neurons for my ANN network. When the training is done they will show this phrases at the bottom of the GUI. What are the meaning behind this phrases and what are the differences between each of them with the normal "perfomance goal met"?

"Maximum MU reached"
"Validation stop"
"Minimum gradient reached"

the other phrases which also appear at the end of training but i already understand is "Maximum Epoch reached" which basically mean that the epoch reach the max value which i have set.