No really I'm serious, I don't have a problem with the pedantry but what is the difference between the ommision of letters, abbreviations and contractions?
I think the apostrophe is mainly used for the possessive, eg Nick's photos. IIRC there are a few common cases where the apostrophe is used to suggest contractions where two words are run together, like isn't, aren't. The word "it's" is a special case, as it looks as if it could be either the possessive ("everything has it's place" - wrong) or the contraction for "it is". By convention it is always used for the latter, and omitted for the possessive. Most abbreviations are indicated by full stops, i.e. and e.g. being examples, but these are often omitted, as in my first example. Many abbreviations, like photo for photograph, are just accepted as new words. Sometimes where parts of a word are repeated, often on repetition, an ellipsis (...) might be used, eg Llanfairpwll....
The "greengrocers' apostrophe" is the mistake commonly seen on greengrocers' advertising boards, of using an apostrophe before the plural form, eg "potato's", "bean's". There are a few cases I've come across where the normal plural form is really uncomfortable, and the urge to insert an apostrophe is almost overwhelming; however I can't think of one OTTOMH!
As Andy says, language is always evolving, but the apostrophe is useful to reduce ambiguity, so we should defend it. And the above may be only partially correct, and certainly maybe partly wrong!