Go's goroutines are not real OS threads but lightweight, virtual threads managed entirely by the Go Runtime. This architectural choice is the secret behind Go's famous concurrency performance. Unlike standard threads that require significant OS resources, goroutines are logical constructs that execute efficiently, allowing developers to spawn thousands of them without crashing the system.

When a Go program starts, the runtime initializes before your code runs, setting up a scheduler and memory allocators. This runtime acts like a mini-operating system. It maps your thousands of goroutines onto a small number of actual OS threads, known as machines, and logical CPUs called processors. This abstraction layer handles the complex work of context switching.

The most critical distinction lies in memory usage. An OS thread typically reserves 8MB of stack space, limiting you to only a few thousand threads. In contrast, a goroutine starts with just 2KB of stack and grows dynamically as needed. This massive efficiency difference is why you can run millions of goroutines on standard hardware.

However, this concurrency model introduces a hard rule: the main goroutine is a tyrant. When the main function returns, the entire process terminates immediately, killing all background goroutines instantly. This behavior forces developers to use synchronization primitives like channels or sleep calls to keep the process alive long enough for concurrent work to finish.