The best way of differentiating between Internal and Inbound links is by thinking about a mailman in a big office building.
They will start the day by sorting through the inbound mail. These are letters that have come from elsewhere, to the company.
Following that they will then begin sorting out the Internal mail. These are letters that are being sent within the business. For example from Jeff in Marketing, to Sandra in Human Resources.
In relation to your website, an inbound link is a link coming from another website, through to yours. This is something that we deal with when building links for you. (Developing a unique article and publishing it on another reputable website, pointing back to a particular page on your website.
Internal links are gateways that connect different pages on your website. You could do this through a blog post about “the benefits of such and such” – and then link through to your services page for more information, etc.