An organism can be defined as living being which can act as an independent unit showing all fundamental characters of life. In simple words organism have organs for different purposes. According to classification organisms are of 2 types. Unicellular (single celled) and Multicellular (multiple cells)
Unicellular organisms are categorized as prokaryotes and multicellular under eukaryotes.
Prokaryotic organism are primitive living beings as they lack true nucleus and eukaryotic organisms are well developed organisms and they have true membrane bound nucleus.
1) Mutlicellular prokaryote: Most bacteria are unicellular, but some bacterial species are multicellular like Myxobacteria. Some species of cyanobacteria are also multicellular like Chara, Spirogyra etc. Magnetotactic bacteria are also multicellular. But some biologists consider these organisms as "colonial", instead of multicellular.
2) Multicellular eukaryote: Most eukaryotic organism are multicellular. Multicellular organisms have well developed body structure and also have specific organs for specific function. Most well developed plants and animals are multicellular. For example almost all species of gymnosperm and angiosperm are multicellular in plants. All animals are eukaryotic in nature and most of them are multicellular.
Unicellular organisms Multicellular organisms
a) Simple body construction (Single cell) a) Complex organization
b) All functions controlled by single cell b) Contains specific organ (specific function)
c) Prokaryotic in nature c) Eukaryotic in nature.
d) Vegetative/Asexual reproduction d) Sexual type of reproduction
e) Few introns in genome e) High introns in genome
f) No cell differentiation f) Differentiation
g) Engulf food g) Specific organ (Food production)
h) Microscopic in nature h) Macroscopic in nature