The four schedules of intermittent behavioral reinforcement are: fixed ratio, variable ratio, fixed interval and variable interval.
In fixed ratio (FR), the reinforcement is given after every x number of times the behavior occurs. For example: when training your do to do a trick, provide her a treat after every fourth correct completion.
In fixed interval (FI), the reinforcement is provided for the first appropriate behavior after a specific interval of time. For example: when training your dog to stay, provided the reinforcement for correct completion only after x amount of time has passed.
In variable ratio (VR), reinforcement is given only after the appropriate response has occurred x amount of times. For example: when training your dog to shake, only provided reinforcement after he has correctly done this x amount of times on the average.
In variable interval (VI), reinforcement is given after for the first correct behavior that occurs after an average of x amount of minutes.