The cell is the fundamental structural and functional unit of all livingorganisms.
The cell theory, first developed in the 1800s, states that all organisms are composed of one or more cells; all cells come from preexisting cells; all vital functions of an organism occur within cells and that cells contain the hereditary information necessary for regulating cell functions and for transmitting information to the next generation of cells.
Organisms vary from single cells (called single-celled organisms) that function and survive more or less independently, through colonial forms with multiple similar cells living together, to multicellular forms in which cells are specialized and do not generally survive once separated. There are 220 types of cells and tissues that make up the multicellular human body.
Most prokaryotes have a cell wall (some exceptions are Mycoplasma (a bacterium) and Thermoplasma (an archaeon)). It consists of peptidoglycan in bacteria, and acts as an additional barrier against exterior forces. It also prevents the cell from "exploding" from osmotic pressure against a hypotonic environment.
A prokaryotic chromosome is usually a circular molecule (an exception is that of the bacterium Borrelia burgdorferi, which causes Lyme disease). Even without a real nucleus, the DNA is somehow condensed in a nucleoid. Prokaryotes can carry extrachromosomal DNA elements called plasmids, which are usually circular. Plasmids can carry additional functions, such as antibiotic resistance.
Some prokaryotes have flagella which enable them to move actively instead of passively drifting.
The cytoplasm of eukaryotes does not appear as granular as that of prokaryotes, since an important part of the ribosomes are bound to the endoplasmic reticulum.
The plasma membrane resembles that of prokaryotes in function, with minor differences in the setup. Cell walls may or may not be present.
The eukaryotic DNA is organized in one or more linear molecules, called chromosomes, which are highly condensed (e.g. folded around histones). All chromosomal DNA is stored in the cell nucleus, separated from the cytoplasm by a membrane. Some eukaryotic organelles can contain some DNA.
Eukaryotes can become mobile using cilia or flagella. The flagella are more complex than those of prokaryotes.
If we see life forms from the point of view of replicators, that is DNA molecules in the actual life, cells satisfy two fundamental conditions : protection from the outside environment and confinement of biochemical activity. The former condition is needed to maintain the fragile DNA chains stable in a varying and sometimes aggressive environment, and probably was the main reason for which cells evolved. The latter is fundamental for the evolution of biological complexity. If we have,let's imagine, freely-floating DNA molecules that code for enzymes that are not enclosed into cells, the enzymes that advantage a given DNA molecule (for example,by producing nucleotides) will automatically advantage also the neighbouring DNA molecules. You can see it as "parasitism by default". Therefore the evolutive pressure on DNA molecules will be much lower,since there is not a definitive advantage for the "lucky" DNA molecule that produces the better enzyme over the others: all molecules in a given neighbourhood are almost equally advantaged.
If we have the DNA molecule enclosed in a cell, then the enzymes coded from the molecule will be kept close to the DNA molecule itself. The DNA molecule will directly enjoy the benefits of the enzymes it codes, and not of others. This means other DNA molecules can't benefit of a positive mutation in a neighbouring molecule : this means that positive mutations give immediate and selective advantage to the replicator bearing it, and not on others. This is thought to have been the one of the main driving force of evolution of life as we know it.
(Note. This is more a metaphor given for simplicity than a possible truth, since probably the earliest molecules of life, probably up to the stage of cellular life, were RNA molecules , acting both as replicators and enzymes : see RNA world hypothesis . But the core of the reasoning is the same.)
Biochemically, cell-like spheroids formed by proteinoids are observed by heating aminoacids with phosphoric acid as a catalyst. They bear much of the basic features provided by cell membranes. Proteinoid-based protocells enclosing RNA molecules could (but not necessarily should) have been the first cellular life forms on Earth.
1665 : Robert Hooke discovers cells in cork, then in living plant tissue using an early microscope.
...I could exceedingly plainly perceive it to be all perforated and porous, much like a Honeycomb...these pores or cells , were not very deep, but consisted of a great many little boxes... – Hooke describing his observations on a thin slice of cork.
1839 : Theodor Schwann and Matthias Jakob Schleiden elucidate the principal that plants and animals are made of cells, concluding that cells are a common unit of structure and development, thus founding the Cell Theory.
The belief that life forms are able to occur spontaneously (generatio spontanea) is contradicted by Louis Pasteur (1822-1895).
Rudolph Virchow states that cells always emerge from cell divisions (omnis cellula ex cellula).