The cell membrane is composed of phospholipids and proteins. The phosopholipids form a double layer of lipid (fat) molecules (on the outside and inside of a cell- hydrophyllic) and are connected to phosphate tails in between the two lipid layers (hydrophobic)- like a jelly sandwich where the lipids are the bread and the phosphates are the jelly. So the layer facing the environment and the layer facing the cytoplasm are exactly the same (hydrophyllic). The charges on the phospholipids (the lipid heads are positively charged [+] and the tails are negatively charged [-]) keep charged ions from passing through the cell membrane (opposite charges repel!). In addition, the lipid heads are round and form spaces between them. These spaces are very small so larger particles and molecules can't get through. All in all, only small, neutral particles can pass through the cell membrane "at will". Substances like oxygen and carbon dioxide are able to pass through "at will".
However, the cell still might need larger particles and/or charged particles- this is where the proteins come in. The proteins have two jobs on the cell membrane. Some of them float around on top or only go partially into the cell membrane. These are marker proetins and tell other things what kind of cell it is. For instance, skin cells bond to each other (to heal a cut) because of the marker proteins which say, "hey, we're the same" and latch together. Likewise, the marker proteins are how your body's substances know which cells are which and how to use them or put them together.
Some of the proteins go all the way through the cell membrane and form a tunnel of sorts. These protein tunnels or channels allow charged and large particles to enter and leave the cell. Some of the proteins require "payment" in the form of energy or ATP. Others are just open and free to use.
(LE) Cytosis: Cytosis is a process whereby the cell membrane wraps around something and brings it into the cell (endocytosis) or stores stuff in a vesicle which merges with the cell membrane to get rid of stuff (exocytosis) from the cell. Endocytosis is like a hug- awwwww and exocytosis is like a burp!