Internal links are referred to the links added from content of the website's internal pages, e.g. the content of about us page may have link to product page you sell. In other words, you are cross linking your website pages from content of the pages. If you open any page of Wikipedia, you can notice many links are available on the page links to other pages of Wikipedia. This website is a great example of building internal links.
Inbound links are those links that you get from others' websites, e.g. I own a website domain.com and on a page of this website, I've added a link pointing to your website. this will be called inbound link. There are also known as Backlinks, Inward Links, etc.