Difference between Relation and term reference field Can anyone explain, what is the difference from practice point of view between relation (by Relation module) and term reference?

I mean if I will use as product attribute taxonomy term, what type of attribute connection I should use?
In your case you should use https://www.drupal.org/project/entityreference

This one is lighter and does the job without overcomplicating. It simply stores 1 to n relations, which may be used later to display in Views.

Relation - this module is much more advanced and allows you to make:


directional references, example


user A is "boss" of user B

different types of references between same entities, example:


reference type 1:  user C is "friend" of user D 

AND
reference type 2:  user C "follows" user D

these references are entities itself, so they are fieldable, could be displayed in Views separately etc.