Link exchange is an off page seo techniques in which we can place our link in other's web site and their's link on our site. There are three types of link exchange:
1 One way link exchange:- In this we can only put our link on other's site and do some other work for them or pay for that in term of money.
2. Two way or Reciprocal link exchange:- I will let you understand it by an example if you site 'A' place a link on site 'B' and site 'B' place a link on site 'A'. Its known also reciprocal link exchange.
3. Three way:- In this type of link exchange site A put a link on site B but Site A doesn't put site B's link on Site A, rather than it will put site B's link on his another site AA.