Hi Sandeep,
We need to define which type of relationship we want to create . Please Follow the below doc for your first question : http://scn.sap.com/docs/DOC-39891.
Card A and Card B are the Cardinality of the source and assigned object respectively. Cardinality basically define how many entities of relation are allowed to be there corresponding to source entity.
e.g. 0..1 ,0..n etc.
Thanks,
Kumar Gaurav