Explain the CAP theorem and its implications.

CAP theorem states a distributed system can guarantee at most two of three properties: Consistency (all nodes see same data), Availability (every request gets response), Partition tolerance (system works despite network partitions). Since partitions are inevitable, choice is between CP (consistent but may be unavailable - MongoDB, HBase) and AP (available but eventually consistent - Cassandra, DynamoDB). PACELC extends this: when no partition, trade-off is Latency vs Consistency.