The job of a semi-colon is quite difficult to describe.  Moreover, he a controversial little guy – some say he has no use whatever, and others could not write without him.

semi-colon
noun
A punctuation mark (;) indicating a pause, typically between two main clauses, that is more pronounced than that indicated by a comma. oxforddictionaries.com

“Here is a lesson in creative writing. First rule: Do not use semicolons. They are transvestite hermaphrodites representing absolutely nothing. All they do is show you’ve been to college.” ― Kurt Vonnegut, A Man Without a Country

“With educated people, I suppose, punctuation is a matter of rule; with me it is a matter of feeling. But I must say I have a great respect for the semi-colon; it’s a useful little chap.” ― Abraham Lincoln

 examples of a semi-colon and correct usage

Here is how to use a semi-colon, named “the most feared punctuation on Earth.” by The Oatmeal (As you can see, semi-colons elicit a lot of strong opinions!!!)
Semicolons are quite difficult to understand, and to use in sentences.  Here is are some guidelines:

Use a semicolon to combine related independent clauses to form a compound sentence.
“Heidi worked out every day; consequently she had constant work as a swimsuit model.”
“I forgot to put eggs in the cake mix; the cake tasted very dry.”

The semicolon is also used to separate items in a list if the list is already using commas. For example:
“The top achievers in the Junior Chess Championships were Josh Harding, U.S.A.; Fabio Linari, Italy; and
Gustav Hahn, Germany.”

If you have 2 independent clauses, that could each stand alone, you may use a semi colon.
“Don’t cancel the restaurant booking; I will change my dental appointment.”

The semi-colon is most commonly used to join two independent clauses,  for a couple of different reasons:

• To make the reader think about the relationship

“The mines were not well maintained; there have been a lot of miners injured on the job recently.”

• To link sentences with a transition signal Using a transition signal such as but, or therefore, two independent clauses are linked to show the close relationship between them.

“The city was on a massive fault line; therefore, the inhabitants were well-versed in earthquake procedures.”

Use the semi-colon when two sentences are too closely related to be separated by a period, or, when you want to form a connection between two independent clauses, or statements, either because they relate to each other or because they are in  total contrast to each other
“One sister adored cats; the other was highly allergic to them.”
You could insert  ‘but’ after the first clause, but the use of the semi-colon here makes the reader think about the connection between the clauses in terms of their contrasts.