The Paleo Diet is largely based on the way people assume that Paleolithic humans ate. It is believed that they were hunter/gatherers that survived on whatever food they found. They were not believed to have domesticated animals, nor farmed crops. For this reason the paleo diet excludes all grains (wheat, rice, etc), legumes (beans), starchy simple carb veggies (white potatoes and corn), and dairy. It promotes eating grass fed or wild and hormone free protein where available, an abundance of vegetables and limited fruits and nuts. Because of all these restrictions Paleo Diets are also gluten free and have been credited with helping people's overall health.
I'm currently doing what is sometimes referred to as Paleo-Low a version of the Paleo diet in which one watches the starchy and sugary carb intake to help achieve ketosis, increase metabolism and thus lose weight. Below is a list of Paleo-low friendly foods. Again, I'm not an expert, nor am I a doctor; this is just my interpretation of the diet. Wikipedia has a great article on the Paleo Diet and there are countless websites out there that can probably better explain it. I hope this helps