Hi Tiffany,
There is a difference between all of these URLs, as far as Google is concerned. If a URL has any one characters different to another, Google considers it to be different. This is true even if the pages each URL loads are exactly the same.
For all of these URLs, you should choose one that you consider to be the "proper" URL. Call this the "canonical" URL - the correct version. There is no gold standard for which versions you should choose, besides for one, which I'll get into later: https://www.abc.com/ is realistically not better or worse than http://abc.com/.
However, you might have a good reason why the entire site should be on HTTPS URLs, i.e. on secured as opposed to unsecured URLs.
Some people choose to not use the "version" of their site that loads with "www" - again, there is no benefit or detriment either way.
For the abc.com/blog/ example, the general rule is that **if more content site beneath the /blog/ subfolder, the URL should have a trailing slash. **If "/blog is just a page with nothing housed beneath it (i.e. there are no pages like www.abc.com/blog/2014/post.html), then you can leave the trailing slash off if you like.
No matter which versions you choose, all alternative versions should be 301 redirected to the canonical version (the one you chose as your preference). If you choose http://www.abc.com/ and someone types in https://abc.com/, they should be 301 redirected to http://www.abc.com/.
The other option is to place the canonical tag on each "alternative" version, pointing to the canonical URL of that page. This means that https://abc.com/, etc. load, but the canonical tag tells Google that the primary version is not on this URL, but on the one you specify in the tag. This is quite easy to do: each URL will be pulling its content from the same file (that is, there are not usually two files for the home page, one populating www.abc.com and one populating http://abc.com - it is the same file being displayed on different URLs). As such, that one file needs to have the canonical tag indicating your desired canonical URL. Each page requires its own canonical tag, indicating the desired URL.
301 redirection to the canonical URLs is the traditional way of getting this done.
Cheers,
Jane
