Relationship basically denotes how activities in a 
networkorder are related to each other.They can be 
classified into 4 categories - SS (Start-Start), SF(Start-
Finish), FF (Finish-Finish), FS (Finish - Start).

For 2 activities say A and B

SS -->> Both A and B start together
SF -->> Start of A coincides with finish of B and so on.

We can also specify the time gap between which these 
relationships work. Say in an SS relationship, there can be 
a 10 minutes gap between start of A and B and so on.

relationship is nothing but reliation between the
activities.    for ex: in any concern so many activities is
there, how to performe the activities is the, question here.
for ex:

let us see:  if u want to built a building, then first u got
a approval from the govet., here 2 activities involved 1)u
built a house 2) u got a government approval,  see here the
logic of relation ship is first u finish ur approval then
only u start a bulding, means here first activite closed and
2nd activity is start, this is called relationship, that too
at what type of relationship is it is.