Stimuli are any events or objects in our environment that elicit sensory reactions from our senses (e.g., sight, sound, touch).
An unconditioned stimulus is a stimulus that naturally triggers a response without prior learning.
A conditioned stimulus is previously neutral stimulus that, after being repeatedly paired with an unconditioned stimulus, starts to evoke a response on its own.
A neutral stimulus does not initially elicit any particular response but can become a conditioned or unconditioned stimulus through association.