2. Use full stops with abbreviations (in an abbreviation the last letter of the word and of the abbreviation are not the same):
etc. (et cetera)
M.P. (Member of Parliament)
3. Do not use full stops with contractions (in a contraction the last letter of the word and of the contraction are the same):
The rule about abbreviations and contractions is not followed by everyone. Sometimes it is a question of style. The important thing is to be consistent.
Note that the word "period" (meaning the punctuation mark) is sometimes used in speech to emphasize something that has just been said. It means something like "That's it! That's all there is to it. There is no need for further discussion." Look at this example:
"I don't believe it was an accident. It was murder! Period!"