There are two general classes of cells: prokaryotic and Eukaryotic. The Evolution of Prokaryotic cells preceded that of Eukaryotic cells by 200 billion years.

• Streptococcus pyogenes, the bacterium that causes infection in the upper respiratory tract (e.g. sore throat) is an example of prokaryotes.
• Yeast, an organism that carries out fermentation and makes our bread rise is an example of unicellular eukaryotes.

Humans, of course, are an example of multicellular Eukaryotes.