What is the exception class hierarchy in Java (Throwable, Error, Exception)?

Throwable
├── Error                     (unchecked — serious, usually unrecoverable)
│   ├── OutOfMemoryError
│   ├── StackOverflowError
│   └── ...
└── Exception
    ├── RuntimeException      (unchecked — programming errors)
    │   ├── NullPointerException
    │   ├── IllegalArgumentException
    │   ├── IndexOutOfBoundsException
    │   └── ...
    └── (everything else)     (checked — recoverable conditions)
        ├── IOException
        ├── SQLException
        └── ...

• Throwable: the root of the entire hierarchy — only Throwable (and its subclasses) can be passed to throw or caught by catch.
• Error: represents serious problems typically caused by the environment/JVM itself rather than application logic — OutOfMemoryError, StackOverflowError, NoClassDefFoundError. Applications generally shouldn't try to catch or recover from these; they usually indicate the JVM is in an unstable state.
• Exception: represents application-level conditions. Splits further into RuntimeException (unchecked — see the checked/unchecked question) and all other checked exceptions.

Common gotcha: catch (Exception e) does not catch Errors (they're siblings under Throwable, not related by inheritance) — you'd need catch (Throwable t) for that, which is almost always a code smell since it also swallows things like OutOfMemoryError that you generally shouldn't attempt to handle.