Separation means to live apart from your spouse. The law does not require you to file documents to live apart and the law does not require you to live with your spouse. To "separate" you technically need do nothing more than move, however, to the extent that separating affects your legal rights, you will want to first consult with an attorney.
A legal separation is where a court has entered an order (much like a decree of divorce) that outlines the rights and responsibilities of each spouse while they are living apart.
Unlike divorce, with a legal separation, you are still married, so neither can re-marry, and the wife may not revert to her former name. After obtaining a legal separation, you have a court order regarding issues such as child custody and support, payment of bills, division of assets, and alimony while living apart. These issues are that same that are addressed during a divorce action except when its over, you remain legally married while choosing to live separate lives.
The process of obtaining a legal separation is the same as obtaining a divorce, and can be just as costly and take just as long, and later, if re-marriage is desired, the parties must go back to court to obtain a divorce.
For some couples, and in certain situations, there are advantages to obtaining a legal separation rather than a divorce. Those advantages include:
It is possible for a married couple to separate and have mutual understanding or even a written agreement without going to court. This is not considered a "legal separation" but can be just as effective. If retention of some of the benefits outlined above are desired, it is wise to retain a lawyer in drafting a separation agreement to ensure that no benefits are lost during the period of separation.
If you decide that a legal separation is best for you at this time, remember that if, in the future you do seek a divorce, that your separation agreement may be binding and may be what the court orders in your divorce.