Cells are the basic unit of life as we know it. All cells have a cell membrane, genetic material, and a liquid inside (cytoplasm/cytosol). Cells may or may not have membrane bound organelles or other structures, depending upon the organism. To learn more about cells, see the links below.
Inside the Cell - This is an excellent book published by the NIGMS. You can view it as a web page, as a PDF, or order a hard copy for free!
Inside a Cell - a nice interactive by the Genetic Science Learning Center in Utah. View an animal and plant cell and click on the organelles to see and hear what they do.
The Inner Life of the Cell - The incredible Harvard animation showing life inside the cell.