In Washington, a couple can choose to file for legal separation or for dissolution of marriage. The paperwork for the two processes is nearly identical, and the outcome for both kinds of cases will include a division of property and liabilities, orders of spousal maintenance (sometimes referred to as alimony), and orders regarding parenting and child support (if there are children).
The major difference between a legal separation and a dissolution is that a legal separation does not end the marriage. It creates legal and financial separation, but the underlying marital relationship continues. Legal separation can be an option when the parties know that they need legal/financial distance without dissolving the marriage. Maintaining the marriage can be helpful to one or both parties in instances where it allows for the possibility of reconciliation, where religious beliefs are opposed to marriage, where one party has a benefit (such as medical coverage) that they are willing to extend to the other, or–as in the military–eligibility for a retirement benefit is conditioned a certain number of years of marriage.
Because there is not the same mandatory waiting period for a legal separation as for a dissolution, couples can get their final paperwork with the court entered more quickly than in a dissolution. Something to be aware of, though, is that it is possible for one person to ask the court to convert the legal separation to a dissolution six months after the final paperwork is entered. So, although a legal separation can be a way to get legal/financial distance and still remain married, there is no built-in guarantee that this “in between” state will last.