Relative (adjective) means: "Considered in relation or in proportion to something else."
In other words, if something is "relative", it means it is dependent on many other varying factors.
For example: people's ideas of what "right" and "wrong" are, are entirely relative; it is different for every person because every person has had different experiences that formed those beliefs. In this case, their beliefs on what "right" and "wrong" are, are RELATIVE to (or dependent on) their experiences.
Another use of the word is when comparing something to a group of its peers. For example: "This phone is relatively old." (It is old compared to other phones out now.)
Related means: "Belonging to the same family, group, or type; connected."
In other words, if two or more things are related to each other, it simply means that they share something in common (a characteristic, an ancestor, a pattern, etc).
For example: A store was robbed, and the only thing stolen was an axe. Later that day, someone found that a tree in the park had been chopped down. One might assume that the two incidents are "related" (ie, the same person did both).
Another example: I am related to my cousin, because we share a common ancestor.
In many cases, "related" can also mean "similar to".
Hope this helps! :)