What is a goroutine and how does it differ from an OS thread?

A goroutine is a function running concurrently, scheduled by the Go runtime rather than directly by the operating system.

• Cost: goroutines start with a small growable stack (about 2KB); OS threads typically reserve 1-8MB. You can run millions of goroutines but not millions of threads.
• Scheduling: Go uses an M:N scheduler that multiplexes G goroutines onto M OS threads through P logical processors. When a goroutine blocks on a syscall or channel, the runtime hands the thread off so others keep running. The OS does not know goroutines exist.
• Communication: goroutines are designed to communicate with channels, following the idea of sharing memory by communicating rather than locking.

var wg sync.WaitGroup
wg.Add(1)
go func() { defer wg.Done(); work() }()
wg.Wait()

Preemption was cooperative before Go 1.14; modern Go preempts goroutines asynchronously so a tight loop no longer starves other goroutines.