What are channels in Go and what is the difference between buffered and unbuffered?

// Unbuffered channel — send BLOCKS until a receiver is ready
ch := make(chan int)
go func() { ch <- 42 }()  // goroutine parks until main receives
v := <-ch                  // unblocks sender
fmt.Println(v)             // 42

// Buffered channel — send blocks only when buffer is FULL
buf := make(chan string, 3)
buf <- "a"  // no goroutine needed — goes into buffer
buf <- "b"
fmt.Println(<-buf)  // a (FIFO)
fmt.Println(<-buf)  // b

// Closing a channel signals: no more values will be sent
jobs := make(chan int, 5)
for i := 0; i < 5; i++ { jobs <- i }
close(jobs)

// Range — exits automatically when channel is closed and drained
for j := range jobs { fmt.Println(j) }  // 0 1 2 3 4

// Comma-ok: detect closed channel
val, ok := <-jobs
fmt.Println(val, ok)  // 0 false

// select — wait on multiple channel operations
select {
case v := <-ch1:            fmt.Println("ch1:", v)
case v := <-ch2:            fmt.Println("ch2:", v)
case <-time.After(time.Second): fmt.Println("timeout")
}