The term "legal" separation typically means a judicial separation, and not all states offer this option. In some states, the process goes by a different name. For example, in Maryland, it's called a limited divorce, and in New Jersey, it's a divorce from bed and board. In any case, the legal process is virtually identical to a divorce. You must file a petition or complaint to start the proceedings, and you must serve a copy of the paperwork on your spouse. You can then try to work out a settlement agreement, resolving issues between you such as custody or property allocation, and the terms can be incorporated into a decree or judgment of separation. If you can't reach an agreement, you'll end up in court where the judge will decide matters for you. In either case, you'll probably spend just as much money and time trying to separate as you would have if you had pursued a divorce instead.
You're Still Married
After you receive a decree or judgment of separation, you're still married. You can't remarry or enter into a registered domestic partnership in states that recognize them. You may not even be free to start dating again, because technically, you still have a spouse. In some states, such as Maryland and Tennessee, it's considered adultery if you enter into a sexual relationship with a new partner. If your spouse decides she wants a divorce instead, she can file on adultery grounds and this might affect issues such as property division or alimony.
Another drawback to legal separation is that it doesn't always terminate either spouse's right to inherit from the other. Claims to a portion of the decedent's estate are still valid because the marriage never legally terminated; you only asked the court for an order dictating the terms under which you would live separate and apart. If you don't want your spouse to inherit from you, this rule could throw your estate plan into turmoil. You can usually protect against it, however, if you include a waiver of inheritance in your settlement agreement. You can also draft a post-nuptial contract in which both of you waive your inheritance rights post-separation.
A legal separation might work well if you want to stay married forever, you just don't want to live together. If you reach a point where you want to end your marriage entirely, however, your state may not provide an option for converting your separation decree to a final decree of divorce. For example, in California, you have to start the whole legal process all over again, asking the court to actually end your marriage this time. This means twice the expense.
In states that don't recognize the judicial separation process, you can still separate in a legal sense while avoiding some of these pitfalls. For example, in Florida, you can enter into a separation agreement, and it's a binding contract after you and your spouse sign it. It's not typically enforceable in family court, but if your spouse breaks its terms, you can file a lawsuit in civil court to compel him to do what he said he would do. You can waive inheritance rights in such an agreement, and you may later be able to simply incorporate your agreement's terms into a final decree or judgment of divorce if you decide to end your marriage entirely. This may depend on your state's rules, however.