There are many excellent books and websites that explain The Paleo Diet clearly in great detail. Do feel free to check out the books and websites listed on Good Reads.
Simply put, eating Paleo is essentially mimicking a hunter-gatherer diet and doing away with harmful additives.
According to The Paleo Diet website,
Fresh fruits and veggies
Nuts and seeds
Healthful oils (Olive, walnut, flaxseed, macadamia, avocado, coconut)
Legumes (including peanuts)
Refined vegetable oils