There are lots of people out there who tend to say that they are divorced, when in fact they are either estranged from their spouses or just separated. There are massive differences among these terms, especially when it comes to certain legal issues, specifically "at fault" divorce states. To understand what those legal issues are in such states, you should first understand what these terms mean on a legal level before you attempt any other relations or relationships with other people.
Estrangement means that your spouse walked out the door years ago and you have not heard from them since. You have no idea where they are, and they have not made any contact with you or your children in all the years since. It is clear that they do not want to remain in the marriage simply by their actions.
However, if you get a divorce because you want to move on to another relationship, you have to make absolutely sure to abstain from a physical relationship until the divorce is final. Trying to claim child support or alimony from an estranged spouse could result in an "at fault" claim from your estranged spouse based upon any current intimate activity with someone else. That means you could be denied alimony, and child support would be difficult to get.
Separation is a legal action taken through the courts. It is often the first step toward divorce. The idea is to give two people some time and space to think things through and decide how best to preserve the marriage. Unfortunately, many partners during separation stray, which ultimately leads to divorce as the faithful partner will not take the other one back.
In an "at fault" divorce state, you can sue for alimony even if you were not married the required number of years. That means that any affair you had while separated jeopardizes your chance of claiming alimony. Even if you and your ex are absolutely certain that the marriage is not reconcilable, you should abstain from new relationships until the divorce is final.
This is the step that completely ends the marriage. Once pronounced, only remarriage overturns it. At this point, you are free to have other partners and other relationships without any ramifications regarding marital property, child support, alimony, etc. If no one was found "at fault" by the time the divorce is pronounced, it is often ruled as an "irreconcilable differences" divorce, and no punitive measures are metered out to either you or your ex. Talk to your separation and divorce attorney about other possible ramifications and what you can do to prevent them.