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Flow control vs. congestion control:

Flow control means preventing the source from sending data that the
sink will end up dropping because it runs out of buffer space.
This is fairly easy with a sliding window protocol--just make sure
the source's window is no larger than the free space in the sink's
buffer. TCP does this by letting the sink advertise its free buffer
space in the window field of the acks.

Congestion control means preventing (or trying to prevent) the
source from sending data that will end up getting dropped by a
router because its queue is full. This is more complicated, because
packets from different sources travelling different paths can
converge on the same queue.