In the context of memory management, what are placement and replacement algorithms ?

Placement algorithms determine where in the available main-memory to load the incoming process. Common methods are first-fit, next-fit, and best-fit.
Replacement algorithms are used when memory is full, and one process (or part of a process) needs to be swapped out to accommodate the new incoming process. The replacement algorithm determines which are the partitions (memory portions occupied by the processes) to be swapped out.