General Categories

There are six major DeFi service categories (source):

• stablecoins
• exchanges
• credit
• derivatives
• insurance
• asset management

Or more specifically, there are several sub-categories of DeFi (source):

• DEXs or Dcentralized Exchanges (Uniswap, 0x)
• Lending/borrowing platforms (Compound, Aave)
• Derivatives (Synthetix)
• Asset management (Mirror, Numeraire)
• Insurance (Nexus)
• Aggregators (Yearn.Finance)
• Asset-backed reserves (Olympus DAO)

Here are full lists of DeFi projects here and further readings here

Stablecoins

It is a type of cryptocurrency without volatility. Tether is one of the largest stablecoins in the world.