One way to think about it is with a mathematical relation, like 'X > Y'. A relational database relation representing this relation would consist of a header tuple, <X, Y>, and a set of tuples whose values satisfy the relation, such as <10, 2>, <8, 3>, <9, 4>. In more common terms, the rows of this table would contain pairs of numbers in which the value of the X attribute is greater than the value of the Y attribute. This table describes the relation(ship) of certain pairs of numbers.

"Each tuple in a relation represents an n-ary relationship...among a set of n values..., and the full set of tuples in a given relation represents the full set of such relationships that happen to exist at some given time--and, mathematically speaking, that's a relation."[1]

[1] Chris Date, Database in Depth, page 46