An internal link is a link to another page on the iamsterdam.com website. An external link is a link to a page outside the website, such as www.ind.nl or www.amsterdam.nl. When adding internal links, they will automatically update when this page is moved or renamed in the Sitecore tree. When adding an external link (or an iamsterdam.com link as an external link) then this link won't update if it gets changed or moved. For this reason, always add iamsterdam.com links as internal links ("Insert Sitecore Link" in the Rich Text Editor of the Content Editor)!