Of these, there are two varieties. Prokaryotes are single-celled organisms that lack a cell nucleus, or any other membrane-bound organelles (the tiny "organs" of a cell). In essence, prokaryotes have all their constituent parts floating around inside their one cell. Imagine an amoeba, for instance.
Eukaryotes are organisms whose cells contain complex structures encased in membranes, such as the cell nucleus. Almost all larger plants, fungi, and animals are eukaryotes. It is assumed that the first living things on Earth were prokaryotes, which eventually evolved into Eukaryotes.
Recently, scientists have sequenced the DNA of a very unusual single-celled organism. It's name is Naegleria Gruberi, and it is a common soil-dwelling amoeba, with a very interesting capability.
"[When] under stress, [It] quickly grows two flagella, like sperm tails, that it uses to swim around. It has a third identity, a hard cyst, that can persist in the soil until conditions become damp and warm enough for it to turn into an amoeba.
"This one-celled organism hunts and eats bacteria as an amoeba, swims around looking for a better environment as a flagellate, and then hunkers down and waits for good times as a cyst," Prochnik said. "It is a very rare process to go from amoeba to flagellate like this." - ( Science Daily )
The image on top is of this amazing amoeba in it's flagellant stage, while the bottom image is of the amoeba in it's normal state. the top image has been colored to designate the different areas (micro tubules are highlighted in green, basal bodies in red, and DNA is stained blue).
It is amazing to see how this amoeba switches from a Eukaryotic state of being to a Prokaryotic state, and back again. This could help shed light on how most of the life we see everyday developed from early single-celled, eukaryotic life, into the multi-cellular, compartmentalized prokaryotic life that abounds today. Cool stuff.