Keywords, meta descriptions, meta keywords

Anybody can explain me the difference in the product information page among the following fields :

• Keywords
• Meta descriptions
• Meta keywords

and how to insert the keywords or meta keywords .
Divided by what ? Comma , semi colon or what ?
there is a limit of characters or I can insert as many as I want.

Meta keywords will probably be removed in the future because Google and Bing are no longer supporting it.

Source: http://googlewebmastercentral.blogspot.se/2009/09/google-does-not-use-keywords-meta-tag.html

As for meta descriptions Google is your friend. Have a look at https://moz.com/learn/seo/meta-description

The keywords are used internally for matching similar products and should be stated with coma separated values e.g. this, that, etc.

The limit for meta descriptions may vary, some say Google's limit for meta descriptions are ~155 chars and head titles are ~70 chars . I found no trustworthy source for this. LiteCart always tend to save a few extra bytes where the mysql column limit for meta descriptions is 256. If not stated it will use the short_description as meta descrption.

LiteCart has no limitation of which characters you may use.

(...)If not stated it will use the short_description as meta descrption.(...)

...and what does the short_description? Is this field is displayed somewhere except the case you described above or $listing_type == row? Is it possible to change $listing_type to row at shop frontend?

Right now, short description is only displayed in the row type listing. You could modify the boxes on the first page accoardingly to the function parameters in func_draw.inc.php.

The Keywords tag is followed by comma-separated keywords that occur on a page, and also display the subject matter of the site. The content of the Keywords tag is not displayed for users. When composing Keywords, the list of key queries must be unique for each page. Usually, the size of the meta tag should be between 2 and 20 phrases, separated by commas.
Source: [url=https://sitechecker.pro/meta-tag-description/]https://sitechecker.pro/meta-tag-description/[/url]