The core difference is memory sharing. A process owns a private address space. Threads live inside one process and share that space, including the heap and global data.
Because threads share memory, they can pass data by just reading the same variables. Two processes cannot; they need pipes, sockets, or shared-memory setups. Each thread still gets its own stack and registers, so it can run its own function calls independently.
The tradeoff is safety versus speed. Shared memory makes thread communication fast but risky, since one thread's bad write can corrupt another. Separate processes stay isolated, so a crash in one usually leaves the others standing. Pick threads for tight cooperation, processes for isolation.
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