There are 3 types of stablecoins currently documented:
Custodial stablecoins are simply backed 1:1 by a centralized entity. For every 1 USDC, Circle holds the equivalent in dollars, and so on with every other coin on the list. This include USDC, Tether, GUSD, BUSD, TUSD
Over-collateralized debt stablecoins are similar to custodial coins in that they are backed by a currency, but in the case of crypto, are often over-collateralized by a highly volatile asset like Ethereum, Synthetix etc. Examples are Dai, sUSD, LUSD, alUSD
Algorithmic stablecoins have a wide array of mechanisms to retain their peg ranging from bond purchases to partial collateralization to programmatic contraction and expansion, or even a combination of all of these. Examples are Ampleforth (AMPL), Fei, Frax, TerraUSD (UST).