Sat is short form of satoshi which the small unit of bitcoin. 1 BTC equals to 0.00000001 sat.
SATS is a BRC20 token that has nothing to do with bitcoin than using bitcoin blockchain for making transaction using bitcoin as fee. In short, SATS is an altcoins that make use of bitcoin network for making transaction.
I meant the altcoins SATS and 1000SATS
1000SATS is an altcoin that is minted under the Binance Smart Chain, which full name is Simon's Cat. I think the major reason for that meme token was to promote and show their support for cats and how they can feed them; the token is listed on major exchanges like Binance and others.
Anytime you see tokens that share the same name with another, you can check the token information on CoinMarketCap or CoinGecko and compare the two to know the difference.