Can anyone elaborate and/or differentiate the difference between the alternate tag and canonical tag?
Its Alt tag which is used as alternate text for image…
whereas, Canonical tag is used to tell the search engine about the preferred path of the page.
The <link rel=“alternate”> tag is used to point to a version of the page that has been translated into another language. For example, you could have
<link rel="alternate" hreflang="es" href="http://es.example.com/">
to tell Google to send queries from Spanish speakers to the es subdomain.
The <link rel=“canonical”> tag is used to give the preferred URL for Google to use when returning the page in the search results. For example, you might have a page with several minor variations, or query parameters that are referenced in a different way in different links. You could then set
<link rel="canonical" href="http://example.com/page?id=product">
to tell Google to use the specified URL in preference to the actual URL of the page.
Don’t be silly … it was perfectly clear from the context that that wasn’t what the OP meant!
I still have a doubt.
There are many countries where English is the Major Language, like United Nations of America, Canada, Australia, United Kingdom. I target some pages with subdomain/folders (geo-target) and all the pages have same content.
Won’t Google consider it spamming and duplicate content.
Should we use both alternate and canonical tag in this case/event.
Off Topic:
United Nations of America … is there something I’ve missed? 
It’s probably sensible to use both canonical and alternate – that way, you can refer to the different localised versions but still leave one particular version as the default/definitive one. That should get around any duplicate content issues.
You can use localised language labels, eg “en-gb”, “en-us” to give country-specific pages even where the countries speak the same language.