A separation, as the name suggests, is a decision by you and your partner to no longer be together. If you are either formally married or living together in a common-law relationship, then a separation (with no prospect of reconciliation) is simply a decision to live separate and apart from each other.
A separation ends a common-law relationship entirely, but for married couples is only the first of several formal legal steps towards getting a divorce.
In contrast, a divorce involves the formal legal end to your marriage, and is essentially a declaration made by a court after defined legal process has been followed. The concept of divorce does not apply to common-law relationships.