Coins vs Tokens
A coin is the native asset of its own blockchain; a token is an asset created by a smart contract on someone else’s blockchain.
In depth
BTC is a coin — it has its own chain. USDT on Ethereum is a token — it lives inside a contract on the Ethereum network and pays fees in ETH. One blockchain can host thousands of tokens, which is why "which network?" matters when you deposit.
Why it matters
Sending a token on the wrong network is one of the most common — and costly — user mistakes.