A channel both synchronizes a pair of threads and passes a value from one to the other. Channels are synchronous; both the sender and the receiver must block until the (atomic) transaction is complete. Multiple senders and receivers can access a channel at once, but a single sender and receiver is selected for each transaction.
Channel synchronization is fair: if a thread is blocked on a channel and transaction opportunities for the channel occur infinitely often, then the thread eventually participates in a transaction.
For buffered asynchronous channels, see Buffered Asynchronous Channels.