In the crypto trader jargon, perps is short for perpetual contract or perpetual swap, a popular derivative product intruduced by BitMEX trading platform. Similar product is now traded also on OKEx and elsewhere.
The perpetual contract is derivative product like a future contract, but there is no expiration date. Therefore you will not get the futures-specific behavior near the expiration date, the perpetual contract behaves more like spot market.
The most obvious difference for you as a crypto trader is that to long a BTCUSD perpetual contract, you deposit crypto, whereas on spot market you would need USD to long a BTCUSD market.
Who are perps for?
If you are speculating short term and interested in getting your profit in crypto, perps will do a better job than margin trading the fiat pair as they pay out directly in crypto.
If you want to long in the sense of buy and hold, do it on the spot market for fiat money.