If you and your spouse just can't get along and find yourselves contemplating splitting up, but you're not quite sure things are rocky enough to actually go through with divorce, you do have options that could help you stay together -- or to make the decision to go through with the final step. Many people talk about being separated and headed for divorce, but this isn't a direct line that you must always walk. It is possible to separate for a while just to see how things go, or go straight to divorce if the situation warrants it.
An unofficial separation is just what it sounds like: You and your spouse will live apart for a while and see how being away from each other affects the tension between the two of you. There can be a brief honeymoonish period where you both start to miss each other a lot right after the separation, so wait that out before making a decision on whether or not to get back together. Make strict rules and cover everything from how long the separation will last at a minimum to how shared bills like costs for your children's schooling or your pet's vet bills will be handled. You can even set limits on contact. But this is unofficial, negotiated by the two of you, and best used when you know there is something wrong but wonder if it's just a rut or if having some space could calm things down.
This is a more serious step because the courts are involved. You and your spouse aren't divorced, but the court sets up a legal separation for the two of you that looks like a divorce in action. This includes an alimony-type monthly payment in many cases. You can use this option if you both do not want to get divorced for whatever reason (such as religion) but when both of you want to legally separate your affairs. This is not an option to take lightly, and using it rests on whether the two of you have a compelling reason to remain officially married on paper.
This is the step to take when you know you can't stay with each other. It's a good idea to get counseling before taking this step, as divorce is pretty much irrevocable, unless you two actually go through the process of getting married again (it does happen). Despite movies and books familiarizing people with scenes of loud fights leading to cries of wanting a divorce, this is a decision that should not be made in the heat of the moment.
If you feel like you just cannot take it anymore, talk to a divorce lawyer about what's involved in the process first. You might find that divorce is the right direction to take, or you could find that you and your spouse may want more counseling first.