An inbound link is a link coming from another site to your own website. "Inbound" is generally used by the person receiving the link. For example, here's an inbound link to our co-founder Dharmesh's blog. Dharmesh could say, "I received an inbound link from HubSpot."
Outbound links are links from your website to another website. The link to Dharmesh's blog we talked about earlier is an outbound link on HubSpot's blog, and an inbound link on Dharmesh's blog.