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What Is DRAM?
DRAM is a type of dynamic RAM memory that utilizes the charge of a capacitor to store ones and zeros. A single piece of DRAM is composed of a large two dimensional array of cells containing ones or zeros that are connected by bitlines and wordlines. Each individual cell can be accessed by utilizing the intersection of a specific wordline and bitline, and reading from or storing to the cell at this address.
In the idealized model, a single transistor and a single capacitor are used to form a cell. When the wordline of the cell is high, the transistor turns on and the capacitor can be written to or read from on the bit line. Handy though this model is for understanding the addressing of the cells, it would not work if it was implemented.

Figure 1
Idealized DRAM, and the problems with it

One problem with the idealized model is that simply having the wordline at a logical one on would fail to turn on the transistor if a logical 1 is stored on the capacitor due to the threshold voltage of the transistor. This problem can be circumvented by the use of a charge pump to bring the wordline to a voltage above logical 1 and flip the transistor on. Another problem is that the capacitors in the cells leak over time, so a refresh routine is needed to periodically read all of the values in the array and then rewrite them. The final problem with the model is that each bitline has parasitic capacitance that causes charge to be stored on the line itself. Thus if a read operation was performed on a cell its charge would be stored along the bitline and cause only a miniscule change in output voltage. In order to solve this problem a sense amp is used.

Figure 2
Realistic DRAM model

In this model the sense amp will clamp the bit line voltage to be a logical 1 or 0 before reading and writing. The process is shown in the following figure.

Figure 3
Flowchart of what happens to the LEDs iin the process of a write and read

This shows the working of a single cell, but during DRAM fabrication there is always the chance of creating a cell that doesn't work. For this eventuality excess cells are created in the array. If a computer tries to read from or write to a broken cell it detects that the cell is broken and rewrites the address of that cell to one of the excess cells.