You now understand NoSQL databases.
Key-value: Simple, fast, time and space per lookup. Use for caching and sessions.
Document: Flexible schemas, JSON-like. Use for content and profiles.
Wide-column: Write-optimized, columnar. Use for time-series and analytics.
Graph: Relationship-focused. Use for social networks and recommendations.
BASE trades consistency for availability. ACID guarantees transactions.
Next, you'll learn how message queues enable asynchronous processing.