Snaps are a device to hold two of something together temporarily as a finished product. Snaps usually come in four sizes.
Ligne 24, Line 24, Durable Dot, or 15mm Snaps are about 19/32 or 5/8 inch. They come in many makeup and finishes and platings and are made by a lot of different onshore and offshore companies for different applications. They can be obtained in various cost configurations to suit the quality and purpose of the end product. These come with various names usually incorporating the company name, like DOT or SK to name a couple, but generally answer to the name snap fasteners. Sometimes the trademark Durable Dot is applied to Ligne 24 by the trade. These are most common in the Boat and Marine trade and can have really tight tolerances, making them difficult to apply and remove (and they are made to be that way). Ligne 24 four part (sometimes called ring snaps) also are available with short, medium, and sometimes long posts. This is very important as the thickness of the material and proper setting of the snap depend on using the proper length post. These also come with a variety of options including "lift the dot" one way snap caps that are good for holster and other applications.
Ligne 20, Line 20, Baby Dot, or 12.5mm Snaps are about 1/2 inch. These are more for the apparel industry but are also used a lot in leatherwork. These are just smaller versions of the Ligne 24 above.
Ligne 16, Line 16, Belt, Glove, Segma, or 10mm Snaps are about 3/8 inch. This is a light duty closure used a lot in the garment industry and also for thinner straps in the shoe and leather. The caps for these can be had in just about any color. They are different in construction from ligne 24 and 20.
Scovill Ligne 14, 15, 16 Gripper Snaps are the little snaps with a wire in them to secure the closure. You don't see these much in the leatherwork trade, but a lot in garments leather or otherwise.
I have found the quality of snaps varies in the myriad of brands out there. Some brands take better (or often only) to machine setting, some work ok for hand setting. Snaps from TLF (Tandy), Weaver, Springfield Leather, Ohio Travel Bag, Zack White, Rochford Supply, Sailrite, Sheridan Leather Outfitters, Standing Bear's, and a lot of little leather shops along the road, will work adequately if not really well when properly set. Note that the marine places will have stainless snaps in ligne 24 only and they will snap and unsnap with extreme prejudice, if not it will seem like you will rip the leather apart, which you may; they actually make tools to get these things apart, and half the time you have to tap them with a hammer to get them to go together.
I can't count the number of hand setting tools I have for snaps. Most of these purchased at a Tandy or other store to do one job one time. This generally means that if I go in the toolbox, I now have about a 50% chance of emerging with the right impact tool to set a particular snap. That being said, if I am in the shop or know I am going out to do a snap job, I use a press. Which one oh god of leather? A Snap N Press from Hoover or the Little Wonder from Weaver Leather. Snap N Press even in the shop if I don't have a billion to do because it is already set-up for Ligne 24 all the time, and I have two of them so one is set-up for one side and one is set-up for the other. Ok if you get up enough nerve to take on a canvas job, and your Snap N Press goes on the road with you, put a lanyard on it and tie it to your belt; just finding the thing, even with SCUBA gear, is a ain't going to happen often situation when it goes overboard. If you have a lot of them to do, the Little Wonder wins hands down, especially because I have two (again one for each side). Setting snaps correctly, no matter what you use is no easy task. If you use the wrong length post, it will never be right, and this is just something you have to experiment with and learn. I recommend using a scrap piece of leather of the same thickness to practice and adjust if necessary. If you have a post that is too long, you can file it back a bit; if it is too short, well you are snapped (or screwed as the case may be), however, once you set, there ain't no unsetting it except with the Dremel. No matter what you use to set the snap, press or impact, do not set too deeply; this will pull the ring or post in on itself and the snap connection (when you snap them together) will be weak at best. Just use firm taps, you can always set it a little more but not a little less, just till it doesn't spin, and getting a mis-set snap out is...well I hope you have a Dremel (have I ever told you that you need to get a Dremel or even a cheap Black and Decker or Wen or something, I have one, it was a bargain on Amazon).