A reaction is immediate and filled with taking everything personal. There is a felt need to control the situation. It arises from ego, putting yourself first. Seen from Dependent Origination, it can be considered the link of Birth of action.
Therefore, it doesn’t provide a space for thoughtful reflection. In choosing to react, one acts out of Craving. When Craving overcomes one, one cannot act with wisdom and understanding. One might choose to cause harm, whether through thoughts, words, or actions.
A response is reflective, arising from wisdom and selflessness, provided through the Brahma Viharas. It provides a pause between what you have received through your senses, and the output you provide to the world. It’s like a little gap, giving a moment to reflect, which leads to a Response without ego.
Here, one acts from wisdom. As such, one is attentive to each situation as it occurs, always being understanding and mindful, speaking or acting out of Loving-kindness, Compassion, Joy, Equanimity or Tranquility. In this case one does not harm another through thoughts, words, or actions.
An acronym that might be helpful to learn to respond, is PAUSE
P =  Patience
A = Analysis
U = Understanding
S = Stopping Thoughts, letting them go
E = Eradicating Reaction