A sentence can be extended with a semicolon when a slight break is preferable to a new sentence. (You cannot do the same thing with a comma. That's called a run-on error, and it's a common mistake.)
Merge Two Sentences with a SemicolonOn occasion, a writer may decide that the next sentence is so closely connected to the previous one that a slight break is more appropriate than a new sentence. A semicolon can be used for this purpose.
slight breaks preferable to new sentences (The semicolons are okay.)
Semicolons Can Replace ConjunctionsSemicolons can be used to replace words like and, but and or. (These are called Conjunctions.)
You can merge two sentences together with a semicolon. The second sentence is usually short and closely connected to the first.
Using a semicolon to extend a sentence is very handy. However, if you find yourself using them regularly, you should probably adjust the style of your writing.
NOT A COMMA
It is possible to link two sentences together with a semicolon. It is also possible to link two sentences with comma + conjunction (e.g. , and, , or and , but).
Here's the key part: You cannot use just a comma to link two sentences. This is a very common mistake called a run-on error. For example:
Extend a sentence
Run-on error with a comma
Using semicolons before conjunctions (and, or, but, etc.)
Using semicolons before transitional phrases (e.g. however)
Using semicolons in lists