Active transport is a method cells use to move individual molecules from areas of lower concentration to areas of higher concentration. This transport is generally from inside the cell to outside the cell or vice versa. Active transport requires energy, since the tendency of molecules in solution is to move from areas of higher concentration to lower concentration.
Active transport is a crucial function of cells, with special molecular machines built into a cell's membranes to accomplish it. Normally, large molecules cannot cross a cell's membranes, but water can. This process of water movement is known as osmosis, which is the diffusion of water across a membrane, and in freshwater environments, cells must have special structures just to get rid of the excess water the solutes in their cytoplasm attract. Diffusion, in general, is the tendency of the components of a solution to achieve an even distribution. Cells, in order to maintain their internal environments, must constantly fight against the influence of diffusion.
Many important functions of cells depend on active transport. For instance, the sodium-potassium pump serves to keep sodium outside of cells and potassium inside, which is crucial to cell chemistry. This results in high excesses of each solute in each destination, and diffusion constantly acts to undo it. These pumps use ATP to accomplish their task.