Some ammo is described as being for "training" and some for "personal defense." What's the difference?
The answer relates to the shape of the projectile itself. Ammunition that is well-suited to training and competition is "full metal jacket," which means that it uses a round-nosed bullet fully encased in copper. It makes a clean hole in the target, and leaves minimal residue in the barrel. That's why it's good for practice.
However, this type of projectile is not ideal for defensive use, because the round nose of the bullet is prone to over-penetration, meaning it passes directly through whatever you're shooting at, and goes on to potentially hit something behind it. In a home, this could mean going through a wall into another room. Outside, it could mean traveling quite a distance down the street or into a building. For that reason, the recommended type of ammunition for defensive use is a bullet with a concave tip or "hollow point." When people refer to JHP (Jacketed Hollow Point) or SJHP (Semi Jacketed Hollow Point), that's what they mean. This bullet design is more expensive because it's more complex to manufacture, but it is designed to "mushroom" when it hits a target, rather than passing through.
Which ammo should I buy for training?
It's a good idea to practice with ammunition that is as similar as possible to what you carry for defensive use, so that the noise level, the recoil, and the weight are all as similar as possible.
In order to determine this, it's helpful to understand how bullet weight is measured. Every round is described with a "gr" measurement (e.g. "115gr" or "124gr"). The "gr" refers to "grains," but that's not grains of gunpowder, it's a rather archaic unit of measurement that is used for the projectile itself. That's why, for example, the "147 gr" round has a lower velocity than the "115gr" round. It's a heavier, slower-moving bullet.
This is good to know because, for example, the 124gr FMJ training round is the same weight of bullet as the 124gr JHP defensive round. In this case, because hollow-point cartridges are so much more expensive, having a target round in the same weight, from the same manufacturer is ideal, because you can get a feel for the load, without having to pay as much. The defensive round is a little "hotter" (more gunpowder for about 100fps higher velocity), so you should practice with it before carrying it. That way, you know exactly what it sounds and feels like when you pull the trigger.