Imagine a giraffe, dandelion, single E. coli, and dragonfly. Life comes in such an incredibly wide variety, but one thing all living things have in common is that they are made up of one or more cells.

If you've learned about cells before, you have most likely heard them referred to as the "fundamental unit of life" - much like how atoms are the basis of studying chemistry and matter, cells are the basis of studying life. Every living thing contains cells that are surrounded by a cell membrane, have DNA, use transcription and translation, are made up of the same molecular building blocks, and use enzymes for their metabolic reactions.

There are three major branches of living things: bacteria, archaea, and eukaryotes. When discussing types of cells, we typically discuss the two major categories: prokaryotes (consisting of bacteria and archaea) and eukaryotes. 

In eukaryotes, there is far more compartmentalization - there are a variety of internal membranes and membrane-bound organelles that undergo specific metabolic processes and enzymatic reactions. This allows for more efficient reactions within the cell due to increased surface area, a reduction of competing reactions, and highly specialized regions.

Prokaryotes, while they may have distinct internal regions with specialized structures/functions, lack membrane-bound organelles allowing for this much greater compartmentalization and efficiency.

There are a variety of differences between prokaryotes and eukaryotes, which can be easily remembered using the acronym, DORA (Dora, Dora, the Exploraaaaa). 

DNA - Prokaryotes have a single, circular chromosome while eukaryotes have multiple linear chromosomes. The eukaryotic chromosomes have introns and are found wrapped around proteins known as histones, while prokaryotic chromosomes lack both of those.

Organelles - Eukaryotes have membrane-bound organelles while prokaryotes do not. Both have ribosomes, but eukaryotic ribosomes are 80S while prokaryotic ribosomes are 70S, meaning that their structures are slightly different and that eukaryotic ribosomes are larger.

Reproduction - Eukaryotic cells reproduce through mitosis and meiosis. As these processes center on the division of the nucleus, they are naturally not used by prokaryotes. Prokaryotes commonly use binary fission as their form of cell division. It is important to note that there is a wide variety of living things, and processes such as budding, fragmentation, and endospores are also reproductive methods, so this section is speaking generally.

Average Size - Eukaryotic cells are, on average, larger.

When comparing the three domains - bacteria, archaea, and eukaryotes - one notices that eukaryotes are more similar to archaea than bacteria, with the two sharing much more similar molecular processes for DNA replication, transcription, and translation than either do with bacteria. This is because eukaryotes evolved from archaea.

An ancient archaeal cell, the proto-eukaryote, engulfed an ancient bacterium and did not digest it. Rather, this bacterium remained in the cell giving it new functionality. Over generations, this engulfed cell lost its independence and became an organelle. This origin of eukaryotic cells is known as the endosymbiotic theory.