A link referring to another page is a backlink (in simple language)
For SEO we look on Internal backlinks and external backlinks
Internal backlinks: links referring to another webpage but same domain are internal backlinks. Example over here: Gundeep Singh Grover (this link takes you to my profile in quora, now the domain is same but the page is different)
External backlinks: Links pointing to another domain example : overview for gundeeep (this is a link pointing to my profile in reddit, now this a external link from quora to reddit)
Now comes DO-FOLLOW and NO-FOLLOW links:
Do- follow links are those which pass the link value or link juice further, to the page where the link is pointed. Like in the 1st example you see my quora link is do-follow link. we can do inspect element to check the link is do follow or no follow.
No-follow links are links which do not pass link value to the pointing link page. These links are favourable to give to the websites which you want to link, but do not take any responsibily of the website youre linking to, like if the website authority is low to which you are pointing you can give no-follow link.
IF in case you have to get link from any site, try getting do-follow link, but this does not meant that you completely ignore it. A no-follow link from a very Authority website is also good.
for beginners i suggest you to just maintain a decent ratio between do-follow and no-follow links, create both of them.
Remember links are to be created genuinely. Do not spam or create unnecessary links. keep things natural.
How to give a do-follow link <a href=”https://abc.com/zy” > click here </a>
no-follow link: <a href=”https://abc.com/zy”>rel=”nofollow”>click here</a>
In the above links the anchor text is “click here”
A back link is further categorized into many categories like contextual links, site-wide links, footer links etc, you will be learning everything further if you learn SEO properly and do fine research.