alts! is a function that accepts a vector of channels to take from and/or channels with values to be put on them (in the form of doubleton vectors: [c v] ). The vector may be dynamically constructed; the code calling alts! may not know how many channels it'll be choosing among (and indeed that number need not be constant across invocations).

alt! is a convenience macro which basically acts as a cross between cond and alts! . Here the number of "ports" (channels or channel+value pairs) must be known statically, but in practice this is quite often the case and the cond -like syntax is very clear.