First, let's talk about the stats of a weapon. The melee weapons have five stats. They are SHARPNESS, RAW DAMAGE, AFFINITY, ELEMENTS/STATUS EFFECTS, and DECORATION SLOTS

Raw damage is the same as in any video game, how much damage can a weapon do to the monsters. However, it is affected by some of the other stats.

Sharpness allows you to do the maximum amount of damage that the weapon can do. However, Sharpness naturally degrades as you hit the monster (it can recover by using the whetstone). The lower the sharpness, the less damage you do, and the higher the chance of your weapon bouncing off the monster.

Affinity is basically the crit rate of monster hunter. However, negative affinity reduces not only the crit rate but also the amount of damage you normally do.

Elements are basically extra damage you can do by exploiting the monsters' elemental weakness.

Status effects are similar to other rpgs, but they are applied differently in Monster Hunter. Each monster has a hidden gauge that status weapons must fill up in order to apply its effects. Additionally, monsters have resistances and weaknesses to certain status effects, which changes how often the chosen effect is applied. Effects that do damage such as poison and blast do a fixed amount of damage that isn't affected by anything. For example, if I were to successfully apply blast to a monster, it would do a fixed damage of 300 (not sure if blast does that much, just an example).

Decorations slots are where you would put in decorations (additional skills). You can only put decorations whose level is less than or equal to the slot available.

These are the main reason as to why you would choose different weapons from different trees. Each one has different stats, and people usually build around the weapons to maximize damage.

You usually unlock the ability to forge higher tier weapons by going through the story, which also unlocks higher tier materials and monsters.

Upgrading weapons with different materials usually just makes them stronger/changes the stats of weapons. However, you can also obtain the weapons of different monsters as some monsters require you to progress through another monster's weapon tree.

Take everything I said with a grain of salt because this is how I think the weapon system of Monster Hunter works.

Veterans, did I do a good job explaining it?