What makes bitcoin safe?
Please read before continuing: Technical Explanation of Bitcoin, Addresses
It may help to introduce a new analogy.
Imagine a massive collection of public lockers placed in the center of the world. Anyone is allowed to take possession of one of these lockers. When someone picks a locker, they are given the only key (or password) that unlocks it. This key is aptly called a “private key”.
When you want someone to send you bitcoin, you ask them to slip some bitcoin in your locker for you. When you want to send your bitcoin to someone else, you unlock it with your private key and place it in their locker!
Here you can see Alice just paid Erin 5 bitcoin. It was simple, quick, and didn't require a middleman. All Alice had to do was supply her private key to access her bitcoin, then she moved it to the locker she wanted to pay. The bitcoin “list” we keep talking about is just a record of all transfers between lockers.
By keeping this record, we always know the balance of any given locker. We can look at the list and see how much bitcoin has ever been placed in the locker, or sent from it.
Also, don’t forget from our lesson on the list. There aren’t actually any names anywhere. The name on the front of the locker is actually your bitcoin address!