Scientists love to categorize the various types of lifeforms on Earth. One of the most basic categories are prokaryotes. This refers to the type of cell that makes up the organism. Most prokaryotes are single-celled organisms, although there are a few multi-cellular prokaryotic species.
Prokaryotes are primarily distinguished from eukaryotes in that they have to nucleus in their cell or cells. There is no membrane bound DNA in prokaryotes. Prokaryotes are generally simpler and less sophisticated than eukaryotic cells. For the part, they don't have either mitochondria or chloroplasts - which are intracellular structures that are involved in making energy in animals and plants. It is believed that they evolved many millions of years before more complex cell types, making them amongst the oldest types of living organisms on Earth.
So what are some specific examples of prokaryotes? The biggest group of prokaryotes are bacteria. The second domain of prokaryotic organisms is the archaea - which was only defined in the late 1980's and early 1990's.
Although prokaryotes are almost always single-celled organisms, this does not mean that they don't group together. In fact, many bacteria are found in colonies. Clinical medicine goes to great lengths to fight infections by large clusters of pathogenic bacteria - all of which are prokaryotes. The biochemical and structural differences between prokaryotes and eurokaryotes (which are what human cells are), is what enables modern antibiotics to function. The antibiotics are designed to kill prokaryotic cells and spare the eukaryotic ones.
Prokaryotes replicated by binary fission. They don't have a means of sexual reproduction. When a prokaryotic cell is large enough and has the means, it simple makes a duplicate of its genome and splits in to two identical cells. Of course, the actual process is a bit more complicated than that, but it's really no more than a basic division.
There are four basic shapes for prokaryotes. The shapes can help scientists and doctors further classify a given bacteria and determine what its biochemical properties may be. The four shapes are; Spiral (as in syphillis), cocci (spherical), bacilli (rod shaped), and vibrio (comma shaped). Often a doctor can tell what a given prokaryote is simply by examining the basic shape, although this is not always the case.
Prokaryotes live just about everywhere on Earth. They have a particular love to water environments. They don't do well if they dry out. They can withstand a tremendous range of temperatures. Prokaryotic cells are typically very small, with their diameters measured in microns. Only the largest can be seen without the aid of a microscope.