What is three way linking,and one way linking and two way

One way link means you have posted or added your link on any web site.
Two way means two websites exchanges their links with each other and some webmasters own multiple sites and try to build up one of their specific sites by offering link exchanges from their other sites. In doing this they add a link to you on their other site and get you to link to the main site they are trying to build up called as three way links.