What are Private Keys?
A private key is a secret number that can used to transfer of bitcoins, encrypt data and more. Each private key corresponds to a public key which is a coordinate on the Bitcoin Elliptic Curve.
Every Bitcoin wallet contains one or more private keys, which are typically generated from a root key, and which are saved in the wallet file. Having knowledge of a private key allows any coins that can be unlocked with that key to be spent, so it is important that these are kept secret and safe.
Private keys themselves are almost never handled by the user, instead the user will typically be given a seed phrase from which their wallet's root key can be derived.
Any Bitcoins sent to the address 1CC3X2gu58d6wXUWMffpuzN9JAfTUWu4Kj can be spent by anybody who knows the private key implementing it in any of the three formats, regardless of when the bitcoins were sent, unless the wallet receiving them has since made use of the coins generated. The private key is only needed to spend the bitcoins, not necessarily to see the value of them.
If a private key controlling unspent bitcoins is compromised or stolen, the value can be protected if it is immediately spent to a different output which is secure. Because bitcoins in an unspent transaction output can only be spent once, when they are spent using a private key, the private key becomes worthless. It is often possible, but inadvisable and insecure, to use the address implemented by the private key more than once, in which case the same private key would be reused.
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