a and b connect the black stones by attaching to a weak stone, . As such they are defensive. For example, b offers white a ponnuki in the center in exchange for security.
c moves out sturdily with the most powerful immediate threat to enclose . This limits white's tactical options.
d moves out faster as part of a more flexible attack. However, black may be plagued by the possibility of cutting the knight's move.
Other supporting black stones in the center will contribute to the threat to enclose at c. With support on both sides, is unreasonable and black could cap.
The one space jump at e is a natural move to move out and attack . Next the opposing c is huge for both players. Considered in isolation, this lets white immediately take c and split black's position rather easily. However, e may have a threat to surround a nearby position, such as the 4-4 stone in a common joseki. Or, e may be sufficiently valuable in reducing white's sphere of influence to justify giving white the large gote reply at c. See also