Cells are the most basic building blocks of all organisms. All living organisms are composed of at least one cell, and complex organisms such as humans are made up of trillions of cells.
There are two types of cells: prokaryotic cells and eukaryotic cells. Eukaryotic cells possess a defined nucleus and membrane-bound structures called organelles, while simpler prokaryotic cells lack these features. The nucleus of a eukaryotic cell contains DNA, and the organelles perform specific functions such as protein synthesis and waste removal. All multicellular organisms are made up of eukaryotic cells; only the simplest life forms, archaea and bacteria, are prokaryotic.