Question: Is there such a thing as an actual "legal separation" where you file legal documents and a petition and such, or are people considered legally separated when they live apart?
Answer: Living apart is different from being legally separated. Although both situations may feel the same, i.e. you and your husband live in different places, the meaning and implications of each choice is different. When you have a signed legal separation agreement that resolves all financial, child and spousal issues associated with the marriage, you have greater legal protection. In addition, you can use your legal agreement to outline rights you want to retain, such as retaining your ownership in the original home, despite moving out.
You and your spouse remain married when you are legally separated, but should you choose to get a divorce, all of the details have already been worked out. Note that states handle the legal separation differently. For instance, in New York, if you have a legal separation for 1 year, then you can file for a no-fault divorce. If you go straight into a divorce without having been legally separated, then you must have a fault divorce.
Check on your state's laws regarding the meaning of a legal separation.