What we call a period in American English is also known as a full stop British English.  The period is the punctuation mark at the end of the sentence.  Here’s a simple definition:
Period or Full Stop
noun
” In punctuation, the full stop (in British English) or period (in American English) is the punctuation mark placed at the end of a sentence. The full stop glyph is sometimes called a baseline dot because, typographically, it is a dot on the baseline.  Periods – Wikipedia en.wikipedia.org/wiki/”

The period is a very simple punctuation mark to use. It breaks up written thoughts at logical points when a complete thought has been expressed and you can now begin another sentences with the next, but related and logical, thought.
A period is used to end a declarative sentence, and secondly, it is used for abbreviations.

Examples of correct period, or full stop, usage in English grammar

1. to mark the end of a sentence when the sentence is NOT a question or an exclamation:
“I am from Japan.”
“The most spoken language of the USA is English.”

2. to indicate an abbreviation
“My first class is at 9 a.m.”

3. ellipses
An ellipses or three dots is used to finish a sentence to show that the sentence is actually partially incomplete, and there is an omission of words, and assumes the reader can conclude the sentence.
“Our Father which are in Heaven…”

4. period following one word
Single words can comprise a sentence, and you use the period accordingly:
“Don’t.”

This example, fromhttp://cybertext.wordpress.com/  http://bit.ly/1xioe9K “show how the placement of punctuation, such as periods, commas, and question marks, can turn something that seems loving and innocent into something more sinister:

Dear John:
I want a man who knows what love is all about.
You are generous, kind, thoughtful.
People who are not like you admit to being useless and inferior.
You have ruined me for other men.
I yearn for you.
I have no feelings whatsoever when we’re apart.
I can be forever happy.
Will you let me be yours?
Gloria

Here are the same words with the punctuation in different places:

Dear John:
I want a man who knows what love is.
All about you are generous, kind, thoughtful people, who are not like you.
Admit to being useless and inferior.
You have ruined me.
For other men, I yearn.
For you, I have no feelings whatsoever.
When we’re apart, I can be forever happy.
Will you let me be?
Yours,
Gloria

The period is a very simple punctuation mark to use. It breaks up written thoughts at logical points when a complete thought has been expressed and you can now begin another sentences with the next, but related and logical, thought. A period is mainly used to end a sentence, and it it is also used for abbreviations.