DRAM or Dynamic Random Access Memory is a type of RAM which holds the data as it is continuously used by a the refresh circuit. Many hundreds of time each second, the content of each memory cell is read and the reading action refreshes the contents of the memory. Due to this refreshing action the memory is called Dynamic and hence DRAM.
DRAM can only hold the data for a fraction of a second and is slower and more complex than SRAM. DRAM takes up less space and are much cheaper.
DRAM's are made from one transistor and one capacitor. The capacitor holds the electricals charge if the bit contains "1" or no charge if the bit is "0". The transistor reads the contents of the capacitor. The charge is held for only a short period of time and then it fades away. That is when refresh circuitry comes in. It reads the content of every DRAM and refreshes them with a new charge before the charge fades away and the data is lost. This is accomplished by "reading every row in the memory chip one row at a time; the process of reading the contents of each capacitor re-establishes the charge."