What kind of errors does this catch?

JSON syntax errors — unclosed brackets, trailing commas, unquoted keys, and similar parse failures, using the same rules as JSON.parse.

Does it validate against a JSON Schema?

No — this checks that the JSON itself is syntactically valid, not that it conforms to a particular schema. JSON Schema validation isn't currently supported.

JSON Validator: Instant Syntax Error Checking

Paste JSON and get an immediate valid/invalid verdict. When it's invalid, the validator points to the specific line and column where parsing failed, instead of leaving you to scan the whole document by eye.