What Are Internal and External Links?
It's good to have a mix. You build internal links when you connect pages from the same website together (interlinking). It is always best practice when you interlink pages that talk about the same topic.
When you give a link to a page outside your website for any reason, this is an external link. Usually, blogs give external backlinks to other sites from their sidebar and their footer sections (these are sitewide links and search engines don't give them much credit). When you insert links in the body of an article, this is an editorial link and passes the most link juice because the writer considered it of high-value to the readers.
Take care with external links; the quantity matters so much; if you give more than 20 from a page that has only a few paragraphs, Google sees it as a low-quality page. If you extend the practice to other pages of your site, again from pages with poor content, Google might think you are building a link farm (spam).
