CSV (comma-separated values) and JSON (JavaScript Object Notation) are both human-readable plain-text formats, which is why they get compared so often. But they are built for different shapes of data, and forcing the wrong shape into the wrong format is where most data-conversion headaches start.
CSV is for flat tables
CSV is rows and columns, nothing more -- every record has the same fields, in the same order, with no nesting. That makes it extremely compact and instantly readable in any spreadsheet program, which is exactly why it remains the default for exporting and importing tabular data: contact lists, transaction logs, inventory counts.
The tradeoff is that CSV has no native way to represent a field that itself contains a list or another object. People work around this with workarounds like pipe-separated sub-values inside a single cell, which is fragile and easy to parse incorrectly.
JSON is for nested structure
JSON naturally represents nested objects and arrays -- a customer record that contains an array of orders, each containing an array of line items, is straightforward in JSON and awkward to impossible to represent cleanly in a single flat CSV file. This is why JSON is the standard format for APIs and configuration files, where the data genuinely has hierarchy.
The tradeoff is verbosity: the same data usually takes more bytes in JSON than CSV, and it is not directly spreadsheet-friendly without flattening it first.
Converting between them
Converting JSON to CSV only works cleanly when the JSON is already flat (or you are willing to lose the nested parts, or flatten them into dotted key names). The json-to-csv converter and csv-to-json converter handle the common, flat case directly in your browser. For checking or cleaning up a JSON file's structure before converting it, the json formatter will validate and pretty-print it so nesting problems are easy to spot before you convert.