What is the difference between hmo and ppo?

What is the difference between hmo and ppo?
 
With an HMO plan, you pick one primary care physician. All your health care services go through that doctor. That means that you need a referral before you can see any other health care professional, except in an emergency. Visits to health care professionals outside of your network typically aren’t covered by your insurance.

PPO plans give you flexibility. You don’t need a primary care physician. You can go to any health care professional you want without a referral inside or outside of your network.  

Staying inside your network means smaller copays and full coverage. If you choose to go outside your network, you'll have higher out-of-pocket costs, and not all services may be covered.

Because it is under the health section, i would say chemo but idk much. i'm pretty sure it's either a machine or a pill. sorry if i am wrong