Jeremy:
When two people have been living together and they decide not to live together anymore, they are separated
When married parents separate, their marriage is not yet ended. Married people need to get a divorce to end a marriage.
Parents who separate often make a separation agreement that says who their kids are going to live with and how they are going to split up the things they own. This is a paper that they both sign.
A separation agreement is not the same as a divorce, but it is a contract, which means both parents have to do what they agreed to do.