All cells in the body of a human being are formed from a single cell, the zygote. In the embryonic stage, active divisions in the embryo form numerous stem cells, from which different cells of the body are formed. Initially all the cells would be similar in make-up, but eventually they differentiate to form different tissues and organs of the body. Differentiating changes the characteristics, structure and function of the cells.
A muscle cell would be different from a cell isolated from the pancreas in many ways. Their functions are different - Muscle cells perform to help in movements and strengthening the body while pancreatic cells produce hormones, enzymes and other secretions. To enable them to function efficiently, they will have characteristic structures too. ie., their cell membranes would have different compositions and shape, the overall shape of the cells may be different, the composition of the entire cellular organelles are different, etc.
But both the pancreatic cell and the muscle cell are similar in the fact that both will be having the same genetic material, ie., DNA inside their nucleus. What makes the two cells different is that, once they get differentiated, some genes in their genetic material are suppressed while some are activated. Thus they look different and function different.