You're wondering, "Is this what happens when one of us moves out of the house?" No. That is physical separation. Physical separation certainly has relevancy in cases involving legal separation, divorce and even unmarried-partner child-custody proceedings, but it is not a legal status in and of itself.
The relationship status of legal separation is granted when one or both of you file something with a court of law asking to be declared legally separated.
In legal separation you do get a "decree," but do not misunderstand: This is NOT a decree of marital dissolution or divorce decree. Meaning, most importantly, that you cannot remarry a new partner.
In most states, the answer is "no." To file for legal separation, at least one of you must have decided that "[t]he marriage is irretrievably broken or one or both of the parties desire to live separate and apart."
In Arizona, interestingly, there is no straight-forward answer to this question. Arizona law is oddly silent on whether the parties must be "living separate and apart" -- the legal term for physical separation -- to become legally separated. However, if you are in a covenant marriage* (applicable only in Arizona, Arkansas, and Louisiana), physical separation can be important. There, voluntarily not living under the same roof for at least two years can get you legally separated (as can a spouse committing adultery; sexual, physical or emotional abuse of the other spouse, a child, or a relative living in the marital household; abandonment for at least a year; drug or alcohol abuse, etc.). Two years of physical separation, or one year after obtaining a legal separation decree, can get you divorced.
Yes, of course. There is no minimum or maximum length of time for legal separation, and a divorce filing will end a separation. If you decide to convert your legal separation into a divorce, the court and your lawyers (if applicable) will use your legal separation agreement as the basis for your divorce agreement and divorce decree.
While statistically that is the most common result, it certainly doesn't have to end in divorce -- which, again, is up to you and your spouse.
You can look at it one of two ways: If you do end up getting divorced down the road, legal separation is like a "dry run" that can prepare you for the divorce proceeding mentally, emotionally, and logistically. The decrees are very similar in terms of the things that the court says must be addressed, including child custody/parenting time/child support/tax exemption distribution (if one or more children are involved), spousal maintenance, and property and debt division. On the other hand, you could argue that you are just wasting time, money and effort by going through what is nearly an identical process twice.
Again, this is a decision for you and your spouse to make based on what is best for your family's own unique situation.
If you are from outside Arizona, and even if you live here, you may not have heard of "covenant marriage." In Arizona, it is a type of marriage couples can opt to enter into that carries certain heightened burdens such as premarital counseling and greater requirements for legal separation and divorce. Please see links below for more information.
Always consult with a competent family law attorney who has experience with legal separations before deciding whether to proceed with one yourself. Best of luck to you and yours!