A spring tide is when the difference between a low and high tide is the greatest while a neap tide is when the difference between the low and high tide is the lowest.
Spring tides occur during the full or new moon, and when the moon, Earth, and the sun are aligned. The collective gravitational pull on the Earth's water is strengthened during the alignment of the moon, Earth, and the sun.
Neap tides occur when the moon and the sun are at the right angle to the Earth, which occurs twice a month. The total gravitational pull on the Earth's water during this time is lesser since the gravitational pull comes from two different directions.