DRAM is Dynamic Random Access Memory.
DRAM is the most common form of RAM.
When someone says that a computer has "one gigabyte of RAM", what they really mean is that the computer has one gigabyte of DRAM.
DRAM is called dynamic because it must constantly be refreshed or it will lose the data which it is supposed to be storing.
Refreshing DRAM consists of reading the contents from the DRAM and immediately writing them back to the DRAM.
DRAM is made up of large arrays of very small capacitors. Each of these capacitors is slowly leaking energy, and if the DRAM is not refreshed, eventually one or more of the capacitors will leak enough energy that a 1 will become a 0 and data corruption will occur.
DRAM is often contrasted with SRAM (Static RAM). SRAM is able to store data as long as power is applied to it, without needing to be refreshed. SRAM is also able to be faster than DRAM. The drawback, of course, is that SRAM is much more expensive than DRAM.
Both DRAM and SRAM lose their contents when the power to them is turned off.