Dry reload means that you also empty the round in the chapter. While try is a simple remove of the magazine and inserting a new one.
Basically the differences in steps:
Dry:
1. Pull the lever on the side to eject the round in the chamber. Keep the lever pulled.
2. Remove the magazine
3. insert a new magazine
4. Release the lever again to load a new round round.
Tactical
1. remove the magazine
2. insert new magazine
A tactical reload is faster but is more prone to cause jams and it only works of course if you have a round left in the chamber. This itself is not the tactical reload. It generally contains every reload "style" to improve speed of the reload. For example taking the magazine out of your vest while simultaneously removing the magazine from your gun.
Obviously now that if you have a 30 round magazine and insert into a empty gun and reload (which can only be dry) you have 1 round in the chamber and 29 left in the magazine itself. If you now tactical reload, you gain 1 bullet since you do not remove the round in the chamber. But when you do a dry reload you do not gain this extra round since you ejected it first. An example for a tactical reload:
It is normally used within Special Forces Operator, such as Navy Seals, Kommando Spezialkräfte, etc.. because it requires a lot of training and normal soldiers usually have the time for a quick reload. You can actually see both being done in PR while there its not possible for you to have the extra round.
EDIT: Just to say that is my knowledge that I gathered playing video games and reading on stuff. I am not an expierenced soldier nor did I ever shot a real gun in my life. So it may not completly correct but if I remember it right AK has some knowledge or at least Stonefly.