Here are some general rules of thumb for distinguishing gowns, jackets, “shortgowns”, and bedgowns. If I were to say “always” or “never”, I’m sure someone would quickly point out a counterexample. But I think you will find that these definitions hold almost all the time.
Gowns are fitted garments consisting of an upper bodice section and a lower skirt section that are usually seamed together, at least partially, at the waist. Most often they are full-length; for Anglo cultures, this means somewhere between low calf and floor length. (It is possible that they were sometimes either cut short or made short to begin with, but more research is needed.) Gowns have set-in sleeves. Often they have cuffs. Gowns, like most 18th century main garments (such as men’s coats), are cut with the “side” seams set toward the back, rather than at the natural side.
For our period (fashion-wise, let’s define it loosely as 1760-1780) there are several variations on this basic theme. The “sack” gown (“sacque” or “robe à la française”) is distinguished by the full, loose pleats that fall from the top of the back neckline to the bottom of the skirts. Even sack gowns are fitted, by means of a lining in back, which is snugged against the body. This style, popular through the first half of the 18th C., was starting to fall out of fashion by our period. Sacks may never have been much worn by the lower classes, as they require a great deal of fabric.
The so-called “English” gown (“robe à l’anglaise”) is fitted in the back by two different methods: 1) by stitched down pleating in the center back section that continues in one long piece into the skirt (“en fourreau”), or 2) by separately cut pieces that are seamed to the skirt. The en fourreau style is earlier, though it remained popular right through our period; the pieced style started to become common in the early seventies.
Jackets are fitted, too, and nearly always have set-in sleeves. Sometimes they have cuffs. Jackets in our period always have skirts (fabric which extends below the waistline), which can end anywhere from high hip to, uh, much lower, but not so long as full length. There is a garment called a “short sack” (or “pet en l’air”), which is exactly like a sack gown only shorter (usually mid-thigh). Was it considered a jacket? A shortened gown? Who knows. Most jackets have set-in waists, but some (like the “caraco” and the BAR’s “Fitted Bodice with Sleeves” pattern) have the skirts cut in one with the body. Maybe “caraco” is just French for jacket, or maybe caracos have a specific cut; more research is needed. Jackets are nearly always cut with the “side” seams toward the back, like gowns and like men’s coats and jackets.
“Shortgowns” and bedgowns are either unfitted, or else loosely-fitted by means of pleating in the back. These garments are T-shaped, with sleeves and skirts cut in one with the body and side seams at the natural side. They generally don’t have cuffs. Except that they are unfitted, they could be very unusual jackets.
Bedgowns maybe were worn in private only, maybe for any “undress” (not formal) wear, and for common women, maybe even in all circumstances. We need more documentation! But it’s pretty safe to say that they weren’t considered a dressy garment.
Riding habits are cut and trimmed rather like men’s coats, though with skirts shaped more like those on a woman’s gown. They are generally about the length of a man’s coat and often (usually?) have a matching petticoat. Maybe they are only an upper class garment, so that a lower class woman would only have one as a hand-me-down or secondhand clothing, but maybe not; more research is needed.
The difficulty in classifying garments
Some garments are exceptionally hard to classify. Is Greenwood’s Jersey Nanny  wearing a “shortgown” or a bedgown? Is Walton’s Turkey Plucker  wearing a bedgown, a jacket (a caraco?), or a gown that is short? In mid-century French paintings by Chardin, such as The Attentive Nurse, Girl Peeling Vegetables, Return from Market, and Grace Before a Meal , are those women wearing bedgowns, jackets, or gowns that are short? We shouldn’t worry too much about it. Not only is our knowledge of 18th century terminology imperfect, but in the 18th century itself, the meanings of these terms were not entirely fixed. We will never pin these terms down absolutely.
Should I Wear a Gown, Jacket, “Shortgown”, Bedgown, …?
A while ago, the “shortgown” was promulgated among reenactors as a universal replacement for the inauthentic “sleeveless bodice”; this was a vast improvement over the previous state of affairs! But since then, various researchers (Karen Mullian, Sharon Burnston, and others) have brought it to our attention that no extant “shortgown” can be solidly dated before about 1780, and that there is no solid documentation that they were worn outside of the Middle Colonies and Quakers and German sectarians elsewhere. Therefore, lacking new evidence, the “shortgown” is probably not a suitable universal replacement for sleeveless bodices.
Furthermore, inventories show gowns to be the most common garment everywhere we look. Inventories don’t address the absolute lowest economic levels, but even runaway ads in the Pennsylvania Gazette show a preponderance of gowns. (This doesn’t mean that everyone was finely dressed. Poor women wore simple gowns of cheap fabric and without trim.)
So do you have to make a gown? Should you make a gown? First of all, how authentic do you want to be? (Remember Bob Sullivan’s immortal definition: Farbs – Those who cut more corners than we do. Authentics – What we do. Damned Stitch-Counting Fanatics – Those that cut fewer corners than we do.) Do you want to be average among reenactors? Above average among reenactors? As authentic as possible according to cutting edge research? Second, how much time and money do you have to spare?
Gowns (worn over stays) would probably be best for almost any time or location near the Rev War. But gowns pretty much have to be individually fitted, and are harder to make than shortgowns. The same is true for jackets (plus I don’t think we really have a firm grasp yet on when jackets are appropriate and which jackets are appropriate for whom). Bedgowns are conveniently unfitted, and although we are not yet certain how much they were worn in public, and by whom, we are pretty sure they were worn in public at least among the lower classes.
Despite recent improvements throughout the reenactor community, you will still be at or above average if you wear a “shortgown”, bedgown, or jacket, regardless of your impression (except for upper class, although jackets do go considerably further up the economic scale).
Also, in my opinion, if you don’t wear stays or jumps, it’s better to wear a “shortgown” or bedgown than a gown or jacket. It’s less obvious that way.
· If you have stays, and are a moderately skillful seamstress (or can afford to pay a good merchant or tailor for an individually fitted garment) then make or buy a gown (best choice) or a jacket (second best).
· If you are a beginning seamstress, or don’t sew at all, make or buy a “shortgown” (mid-Atlantic region) or bedgown (any region), and if possible, wear the “shortgown” or bedgown over stays or jumps.
· If your impression is of a wealthy woman, you really must wear a gown or jacket (or riding habit) over stays or jumps, rather than a shortgown or bedgown.
You may also want to look at the other people you tend to reenact with. If everybody else has a gown, for example, maybe you should wear a jacket to spice things up!
 “Shortgown” is strictly a reenactor term. (The BAR pattern refers to it as the “American Shortgown”.) The period spelling seems to have been consistently “short gown”, two words, and it is difficult, if not impossible, to distinguish between the phrase “short gown” meaning the distinctive garment described above, and the phrase “short gown” meaning a regular gown which is simply shorter than full length.
 See The Mythical "Bodice" by Ingrid Schaaphok, Salmagundi, Nov-Dec 1999, for discussion of the “sleeveless bodice”.
 Via the 18cWoman mailing list (home page: http://www.egroups.com/group/18cWoman)
 The phrase “short gown” (two words) does appear before 1780 and/or outside of Middle Colony/Quaker/German contexts. Does it refer to what we call a “shortgown”? Does it refer to other garments? No documentation has yet surfaced to make that clear.