What are the similarities and deferences that exist in a paragraph and an essay

Besides the obvious difference of length, there are fewer
disparities in the concept of writing a paragraph and an essay and more similarities. 
In fact, there is a form that Edward P. Bailey and Philip A. Powell in The
Practical Writer term a "One Paragraph Essay."  With such a paragraph, the
writer creates a topic sentence that in its general statement contains an opinion.  This
opinion, then, is supported with any of the following; a quick example, a narrative
example, a statement by an authority, a statistic. As a conclusion, the writer rewords
the topic sentence with what is called a clincher, a thought-provoking remark. Here is
the outline for such a
paragraph:

• Topic
  sentence


• Specific
  support


• Specific
  support


• Specific
  support


• Reworded topic
  sentence with clincher 

Thus,
the concept of a paragraph is to state to the readers what you are going to tell them,
then tell it to them, and then, reiterate what you have just told
them.

Essentially, an essay has the same purpose, just in
greater depth and extent.  However, in a five paragraph essay, for instance, there is an
introductory paragraph which is not included in the writing
of a single paragraph.  In the introduction, the thesis is similar to the topic
sentence, but it contains 3 opinions on the general statement, rather than
just 1. 

Of course, the three body paragraphs
are much the same as the one-paragraph except that they do not have a
reworded topic sentence as their last sentence as they are mainly the supporting
paragraphs.  Instead, the last sentence is usually a transition sentence that includes
points from the paragraph that can lead to the next paragraph.  Thus, the writer
connects an idea from the written paragraph that will launch the next
paragraph.

Finally, the last paragraph is the
concluding paragraph.  So, instead of one sentence that
rewords the topic sentence as in a paragraph, the writer ties all the main ideas of the
body paragraph together and then rewords the thesis.  Again, a clincher ends the essay
just as it does the paragraph.

Here is an outline of a
five-paragraph
essay:

• Introduction
  with motivator and thesis statement containing 3
  opinions


• First Central
  Paragraph with topic sentence, and specific supporting sentences (4-5)
  and transitional sentences. Transitional sentence to the next
  paragraph


• Second Central
  Paragraph [same pattern as the
  first]


• Third Central Paragraph
  [same
  pattern]


• Conclusion with
  reworded thesis statement and clincher

So, the
paragraph, in essence is a mini-essay.  They are similiar
in that they have the same purpose of communicating an idea with support.  Here are the
differences: 

1. Whereas
   a paragraph has a single topic sentence with a single opinion, an essay has a thesis
   statement that contains three opinions. 


2. Whereas a
   paragraph has a support as a single sentence, in an essay, a support is an entire
   paragraph. 


3. And, whereas a paragraph has a single
   reworded topic sentence as its concluding sentence, the essay has a
   paragraph.

Source:  Bailey, Edward P. and
Philip A. Powell, eds. The Practical Writer.  Boston:  Thomson
Higher Education, 2008.