Main different between outbound link or inbound link?

Well, I guess from the term everything clears up Outbound Link: They are links going out of your site, like you have a website acdc.com (you share information about various business sites to user) you add xyz.com site over to your site not your site acdc.com is pointing xyz.com, it's termed as outbound link.
Inbound Link: They are the links coming over your site like xyz.com sees that you have added them over their list and they write a post about it in their blog and add your site referece acdc.com over, xyz.com/blog (say) now such type of links are termed as inbound link.