In the days of print publication, authors recieved a number of "pre-print" copies of their journal articles, run off before the final, softbound copies of the journal were printed. These copies were free for the author to send out to other scholars. 

• As photocopying became more affordable, scholars were able to copy these pre-prints and send them out to other scholars who might request them. But this was limited to the 'invisible college' of scholars who knew each other well enough ot make those requests. 
• Early adopters of the Internet, such as physics and mathematics scholars, saw that online archives of electronic preprints could be made available through the web, and preprint hosts such as Arxiv.org were born.  
• Hosting e-copies of the pre-print versions of your publications is now known as self-archiving, or, sometimes, 'green' open access.

What is a pre-print?

Pre-prints can be:

1. A draft copy of an article that you share with colleagues to get informal feedback. 

2. A copy of the pre-peer-review, pre-corrections, or just pre-publication (sometimes called 'post-print')version of the article.
   Downloading a copy of the printed PDF and sharing it is usually forbidden by the copyright agreement authors sign, but a significant number of publishers allow the posting of pre-publication or pre-corrections versions on open access repositories.

3. In the old-school print world, the terms 'pre-print' and 'off-print' were used for author's copies--  typeset copies of articles published in printed journals that were distributed to the authors for distribution to colleagues. Scholars sometimes use that terminology today, but publishers now prefer the term 'Publishers version/PDF' and generally disallow the self-archiving of these versions.

Open Access journals don't charge a subscription fee to read/view their contents.

• Some open access journals are supported by institutions or scholarly societies as part of the ongoing cost of operating-- they charge no fees to authors or readers. (Sometimes called Platinum OA or Gold-NoAPC journals)
• Some open access journals charge a fee to authors  (APC: Author's Processing Charge) in order to support the costs of publication (internet access, server space, phone calls to reviewers, paper clips). No fee is charged to readers. (This is sometimes called Gold OA or Gold with APC) These fees can vary from around $300 to upwards of $5000, depending on the publisher. Scientists, who may be required to post their research in open access forums by funding agencies, often build the cost of OA publishing into their grants or funding applications.
• There are even hybrid OA journals, where a for-profit publisher gives authors the option of paying an OA fee in order to have their particular article Open Access, while the rest of the journal is restricted behind paywalls.

Check out the Directory of Open Access journals:  https://doaj.org/

This chart from the University of California Summarizing Open Access Strategies gives a good summary of the differences between the two approaches and the Self-Archiving (Green) option discussed above: https://libraries.universityofcalifornia.edu/groups/files/about/docs/UC-Libraries-Pathways%20to%20OA-Chart.pdf