Weapon Proficiency Steps

The level of skill with which a character fights is divided into six general categories: non-proficiency, familiarity, proficiency, expertise, specialization, and mastery.

To summarize:

Unless you are a Fighter, the default state is non-proficient. It means you can’t use the weapon in question, and if you do, you’re terrible at it. Familiarity is only for weapons that are similar to ones you know how to use. These levels of proficiency don’t require slots. They are greater and lesser penalties.
If you invest a weapon proficiency slot in a weapon, you become proficient (fighters are proficient in all weapons by default and don’t need to invest slots to reach this level). All related weapons (of the same group) become familiar to you. Warriors invest one slot to become proficient in a whole group of weapons. This is also the cap for all non-warrior classes.
Warriors can invest slots into becoming experts in individual weapons. Fighters can spend one slot to become experts in whole groups of weapons.
Only Fighters can specialize in individual weapons. Multi-class Fighters can only specialize in one weapon, and cannot master.
Single class Fighters can become Masters and above in one weapon.

Many weapons are very similar in construction and techniques of use; for example, using a bastard sword with one hand is not too much different from using a long sword. Both weapons are heavy, two-edged blades that rely on slashing or chopping strokes to cut through armour. Weapons are divided into Weapon Groups according to these similarities. All weapons in a proficiency group are considered to be related to each other.

Non-Proficiency
Familiarity

All Classes:
Proficiency

Warriors Only:
Expertise

Fighters Only:
Specialization
Mastery
High Mastery
Grand Mastery

Weapon Proficiency Steps

Forgotten Realms: Birthright Avanpallandt