Cells are the building blocks of living creatures and are on a level of organisation between the cells molecules and tissues or whole organisms. In order to understand the molecular workings in cells and tissues it is very important to have a basic understanding of cells as a whole.
 Cells in plant and animal tissues. (A) Cells in a plant root tip, each surrounded by an extracellular matrix (cell
wall), stained red. (B) Cells in the urine-collecting ducts of the kidney. Each duct is made of closely packed cells (with nuclei stained red) that form a ring. The ring is surrounded by extracellular matrix, stained purple. (B, from P.R. Wheater et al., Functional Histology, 2nd ed., 1987, Churchill Livingstone.) Figure 1-2 from Essential Cell Biology, by Alberts et al., 1998, Garland Publishing Inc.
Cells can be seen as the result of evolution by natural selection. Molecules started to replicate, produce other molecules and then a membrane of lipid molecules was formed. This enabled the early cells to produce and maintain an internal environment that is different form the outside environment.