The value of memory_target specifies the maximum size of memory used by Oracle for SGA and PGA.
The total amount of memory used can exceed the specified value, for example for memory allocated for PL/SQL tables and varrays. Memory used for such purposes is allocated as long as there is memory available from the operating system.
The value of memory_target controls, together with memory_max_target, the default value of sga_max_size
See also
v$memory_target_advice gives a rough estimation on the influence of different values for memory_target on DB time.