I have contacts appearing in my iPhone that I didn't put there and don't know. How is this happening?
Possibly you have a service enabled on your iPhone which adds another set of contacts. (It might be Facebook contacts or whatever else.)
Check the Groups in Contacts and try switching the different groups off to see which one's adding the contacts you're seeing. Once you know which group is responsible for that, you can clean that up.
Are you sharing your Apple ID account login information? If Facebook and Twitter hasn't added contacts to your device then check and see if you have multiple devices using your account.
People share accounts all of the time, and Apple highly recommends that you don't for these reasons.