In behaviorism, excitatory conditioning occurs when a conditioned stimulus (CS) has a positive relationship with the unconditioned stimulus (US)--essentially the CS eventually predicts the occurrence of the US. In this circumstance the CS begins to elicit the behaviors associated with the US.
This is considered the most common type of conditioning and is considered easier than conditioned inhibition.
Examples of excitatory conditioning include fear conditioning and taste aversion (also known as the Garcia Effect). This is when after eating or drinking something that makes us sick we no longer want to eat/drink that substance and even the thought of it makes us ill.