The J curve represents population growth with no restrictions, while the S curve represents population growth with a restricting factor. Exponential and logistical are the two modes of measuring population growth.
The exponential, or unrestricted, growth is portrayed by the J-shaped curve of population increasing over time. The logistical, or restricted, population growth has numbers accelerating to the point of maximum growth and then decreasing over time, forming an S-shaped curve on a graph. The logistic growth starts by the population increasing to meet its resources, stabilizing at the resource's maximum growth and decreasing as the population exceeds its available resources. Population grows as limits are removed and declines when new restrictions emerge.