Antique 19th and early 20th century bottles have long attracted collectors, due to their beautiful shapes and colors, historical interest, and availability to anyone willing to do some sleuthing and some digging.
The rarest and most prized bottles include mouth-blown (as opposed to machine made) bottles from the 1800s, especially those in interesting shapes (e.g. figural bitters), with dire warnings (e.g. medicine or poison bottles) or other interesting embossings. Collectors often seek pontil marks and rare colors, and of course, condition is paramount.
There are in fact dozens of categories of collectible bottles, including milk bottles, soda bottles, beer bottles, flasks, ink bottles, perfume bottles, fruit (Mason) jars and Ball jars, liquor bottles, snuff bottles… the list goes on.
Many collectors also focus on specific brands or regions (e.g. the West), or bottles that were used in certain venues (e.g. pharmacy).