Any fair reply to "What is a bitcoin wallet?" starts with two older coins that casino cashiers often list beside Bitcoin, Ethereum and Litecoin, which do quite different jobs. Ethereum runs programs and carries tokens, while Litecoin mostly moves value from one address to another. Knowing how each network charges its fees and what its addresses look like keeps a transfer on the right chain, where a mistake can cost the whole amount. Nothing here is a reason to buy either coin.
The Ethereum network measures the work behind each transaction in units called gas. Ethereum gas fees are paid in ETH, and the total depends on how much gas an action uses and on the price per unit at that moment, which climbs when the network is congested. A plain transfer needs less gas than a call to a complex contract. Wallets show an estimated fee before anything is confirmed, and on a busy day that fee can exceed a small transfer. A transaction that runs out of gas fails and still pays for the gas it used up.
Ethereum is a public blockchain whose native coin is called ether, usually written ETH. Unlike Bitcoin, the chain does more than move its own coin: it runs small programs called smart contracts, and many tokens live on it, including the ERC20 version of USDT. Every action on Ethereum is paid for in ETH, even when the thing being sent is a token. A wallet that holds only USDT on this chain cannot move it without a little ETH alongside. The price of ETH moves up and down like any other coin.
A Litecoin wallet works much like a Bitcoin wallet, but the addresses look different. A Litecoin wallet address commonly starts with L, M or ltc1, and most wallets refuse a Bitcoin address in that field, though care is still needed. When Litecoin is withdrawn from an exchange or casino, the right choice is the native Litecoin network rather than a copy of the coin issued on another chain. Sending from a personal wallet means pasting the address, reading the fee and confirming the amount, because a confirmed transaction cannot be reversed.
Answers in brief
How does Litecoin compare with Bitcoin?
Litecoin produces blocks about four times as often and tends to charge lower fees, though its price swings like any other unstable coin.
What decides Ethereum gas fees?
The amount of gas an action uses and the price per unit at that moment, which climbs when the network is crowded.
Why does a wallet need ETH to send USDT on Ethereum?
Gas on Ethereum is paid in ETH even when the coin being sent is a token, so a wallet holding only USDT cannot move it.
What is Ethereum, and what is ETH?
Ethereum is a public blockchain that runs smart contracts and carries tokens; ETH, or ether, is its native coin and pays for every action on it.