What are clock domain crossing issues and basic synchronization techniques?

Clock domain crossing (CDC) occurs when a signal passes between circuits clocked by different clocks, risking metastability if the signal changes near the receiving clock edge. Basic synchronization uses two or more flip-flops in series (synchronizer) to allow metastability to resolve. This adds latency but reduces failure probability exponentially with each stage. For multi-bit signals, simple synchronizers cause data incoherency; solutions include Gray code encoding for counters, handshake protocols, or asynchronous FIFOs for bulk data transfer.