If you take a look at your home you will notice it is enclosed by outer walls. All cells are enclosed within something called a plasma membrane. The plasma membrane is not exactly the same thing as the wall in your house, but it does hold parts of a cell inside. These parts of the cell are what biologists call "organelles." That is a Latin name for little organs.
Sometimes people think of cells as a balloon filled with fluid. That is not really true because a balloon does not let things move in and out like the membrane of a cell. It is important for cells to be able to move materials in and out of the cell.The plasma membrane in cells have special structures that allow water and other food materials to pass in and out of the cell. At thousands of places across its surface, the plasma membrane holds gatekeeper structures- called channels and pores. These channels allow things to move in and out of the cell. Not everything can freely pass in and out of the cell. The cells allow only those things which are necessary for them to function.
Cells are amazing. They are all made of similar building blocks, but they do many different things depending on how they are programmed. Some cells carry oxygen to parts of our body. Other cells defend against invading bacteria and viruses. There are cells that transmit signals through out the body like the signals from your eyes to your brain while reading this article. Some cells can even convert the sun's energy into food. This is called photosynthesis. There are hundreds of jobs that cells can do. Cells also make other cells in a process called cell division. That is something other building blocks cannot do.