Basic PC memory unit is RAM (Random Access Memory) which, as the name states, can be accesses at any time, by any process of the PC as long as its not fully occupied. However it is accessed in burst of info with blanks in between of one another. To ensure seamless data stream from and to a process most of the OSs use also "VIrtual Memory".
Virtual Memory is a way to put parts of data on PC's HDD (Hard Drive) so that a process can operate without any memory shortages and problems. It can also store excesive data that cannot fit in to RAM due to its shortage, but we need to remember that a HDD being over 100 times slower than RAM, makes a PC very slow in those kind of situations.