Hi Amir,
http://www.learnersdictionary.com/blog.php?action=ViewBlogArticle&ba_id=93
http://forum.wordreference.com/showthread.php?t=29820
http://answers.yahoo.com/question/index?qid=20080218015002AAhW3hk
http://english.stackexchange.com/questions/15208/relation-versus-relationship
http://www.english-test.net/forum/ftopic26088.html
http://www.englishforums.com/English/RelationVsRelationship/bnvhg/post.htm
http://www.italki.com/question/84726
Relation: In the basic form it means your relatives; mom, dad, grandma, brother, cousin, etc.
Relationship: This is an interaction between two or more people. You have relationships with people everyday; at work, at school, at home and nearly everywhere. If you are a man and you really like a woman but aren't having s-e-x, you have a Platonic relationship. If you only buy office supplies at one store you can say you have a business relationship with them.
Altogether, I think 'relation' mostly refers to 'existence' of a link, connection or let's say bond between two or more people, countries, things or events but 'relationship' might talk about the 'quality' of such a connection. Bear it in mind that context plays a substantial role in determining whether or not they can interchangeably be used.
Typical example: What is their relation? They are siblings.
How is their relationship? They are very supportive to each other.