This punctuation mark is a period (American English) and a full stop (British English).
It comes at the end of a sentence, immediately after the last word, with no space. After the period there is one space before the next sentence begins with a capital letter.
However if the period comes at the end of a paragraph, then the next sentence will start on the next line.
A period mark is also found in some other situations:
a) In internet and e-mail addresses, where it is called a dot, for example email@example.com (my private e-mail address which you can use at any time).
b) In numbers, where it is called a decimal point, for example 3.7, which is “three point seven.”
c) In money it has a similar use, for example $6.25, which is “six dollars and twenty-five cents,” or “six twenty-five.”
If you know of any other situations please let me know.