It consists of an array of small capacitors; the presence, or absence, of a charge in a particular capacitor represents the storage of a '1' or '0' bit. Over time, the charge dissipates, so it must be refreshed on a regular basis; typically in the low numbers of milliseconds. DRAM is also volatile; it loses it contents if power is cut off.
Contemporary DRAM is implemented in integrated circuits, but very early DRAM implementations used discrete components. The very first digital electronic computing device, the Atanasoff–Berry Computer, used a form of DRAM for its memory.