The way ive been compartmentalizing it is this:
Paleo is a way of eating, and by itself tends to be a little more strict (especially in terms of dairy and salt).
Primal is a way of life, and yeah that sounds cheezy but its really that. It takes the main tenets of paleo but makes them incorporate-able into an average lifestyle, and allows for variations without beating one up about it. Moreover, it uses the same logic and science behind the diet to support aspects of the lifestyle, such as getting proper sleep, lots of low-intensity exercise, sunlight, play, etc etc etc.