Many people are confused about what is meant by "separated" -- and it's no wonder, given that there are four different kinds of separations. However, how a couple is separated can have important affects on property ownership:
Trial separation. When a couple lives apart for a test period, to decide whether or not to separate permanently, it's called a trial separation. Even if the spouses don't get back together, the assets they accumulate and debts they incur during the trial period are usually considered marital property. This type of separation is usually not legally recognized, but is instead a specific period in a couple's relationship.
Living apart. Spouses who no longer reside in the same dwelling are said to be living apart. In some states, living apart without intending to reunite changes the spouses' property rights. For example, some states consider property accumulated and debts incurred while living apart to be the separate property or debt of the person who accumulated or incurred it. In other states, property is joint unless and until a divorce complaint is filed in court. Also in some states, couples must live apart for a certain period of time before they are permitted to file for a no-fault divorce.
Permanent separation. When a couple decides to permanently split up, it's often called a permanent separation. It may follow a trial separation, or may begin immediately when the couple starts living apart. In most states, all assets received and most debts incurred after permanent separation are the separate property or responsibility of the spouse incurring them. However, debts that happen after separation and before divorce are usually joint debts if they are incurred for certain necessities, such as to provide for the children or maintain the marital home.
Again, a couple's decision to permanently separate may not be considered a legal one unless one party files for legal separation instead of divorce.
Legal separation. A legal separation results when the parties separate and a court rules on the division of property, alimony, child support, custody, and visitation -- but does not grant a divorce. This isn't very common, but there are situations where spouses don't want to divorce for religious, financial, or personal reasons, but do want the certainty of a court order that says they're separated and addresses all the same issues that would be decided in a divorce.
The money awarded for support of the spouse and children under these circumstances is often called separate maintenance (as opposed to alimony and child support). In some states, separate maintenance can be obtained with a motion pendente lite, or a motion "pending the litigation." Usually a lawyer files this motion. These motions set the tone for what may be awarded in a future divorce judgment.
To learn more about separation and divorce, see Nolo's Essential Guide to Divorce, by Emily Doskow (Nolo).