"relation" is a connection between things or people.
"relations" can refer to interaction between more than two people or between two groups of people. "Relations between the two countries was at an all time low."
It can also refer to relatives, family. "We're visiting relations up North this weekend".
Sometimes "relationship" is used where just "relation" would be enough. ("There is a relationship between inflation and interest rates" - "relation" would have been most correct.
"Correlation" is used to describe a numeric relation between two or more numbers. When one number ("temperature") goes up, another one "school attendance" might might go up or down.