What is the difference between SQL and NoSQL databases, and when do I choose each?

Default to SQL (Postgres) unless you have a specific reason not to. Here's the actual decision logic: SQL shines when your data has relationships — users have posts, posts have comments, orders have items. That's… almost every app. You get joins, transactions and constraints that stop bad data at the door. NoSQL (document stores like Mongo) shines when your data is genuinely schema-less or you need massive horizontal write scale — event logs, product catalogs with wildly varying attributes, caching layers. The mistake beginners make is picking Mongo because 'JSON is easy', then re-implementing joins and constraints badly in application code. If you can draw your data as a table with lines between tables, use Postgres.