Checked: the caller can recover, so the compiler nags. Unchecked: fix the code, so no nagging.

Checked exceptions are Exception subclasses outside the RuntimeException family, and the compiler forces every caller to catch or declare them. RuntimeException and its subclasses are unchecked and usually mean a bug, such as a bad argument, a null or an illegal state. Parsing a number badly also lands in the unchecked family. Errors describe a broken JVM and are rarely worth catching.

Read it once, then a question tests whether the trick stuck.