Exposed terminal
[figure from https://en.wikipedia.org/wiki/Exposed_node_problem]
S1 is transmitting packages to R1, S2 is prevented from sending packages to R2, because the carrier sense tells R2 it will interfere with the transmission of S1. But in fact, such an interference would not occur because S2 is sending packages to R2, not R1.
Hidden terminal:
[figure from https://en.wikipedia.org/wiki/Hidden_node_problem]
There is a transmission from A to B. At the same time C attempts to send packages to B. Because A is outside of the signal range of C, the carrier sense will tell C it's OK to initial a transmission to B. But in fact, such a transmission will interfere the transmission from A to B. In this case, A is called the hidden terminal of C.