Bitcoin is stored in a Wallet, of which there are three types: Cold Wallet, Hot Wallet, and Paper Wallet. There isn't much between the trio other than the former being the most secure (though it's only as safe as the environment it's stored in).
Here's the low-down.
A Cold Wallet is a Bitcoin Wallet that's stored offline on a device that's not connected to the Internet when it's not in use. That could be a digital ledger, like the much-loved Ledger Nano S; an external hard drive, such as a USB stick; or something similar.
A Hot Wallet is a Bitcoin Wallet that's located on a device that is connected to the Internet at all times, like a computer, smartphone, or tablet. More often than not, such storage solutions — Coinbase Wallet, for example— are based in the cloud.
Related: How To Mine Your First Bitcoin
A Paper Wallet is a Cold Wallet that's kept offline, wherein both Private and Public Keys are printed on a piece of paper, written on a Post-it, or stored on another physical medium (if you wanted to be extra secure, you could encode them in a microdot).