Reeks of Monsters--
Although it's never explicitly explained (like so many other systems within Bloodborne), all enemies and bosses within the game are classified as one of three types. The three classes are Beast, Kin, or what refer to as Neutral. It's very possible to beat the game without ever knowing these classifications exist, but you'll be missing out on a lot of interesting strategies and decision making. Also on a super basic level, the damage to Beasts/Kin blood gems are extremely powerful while simultaneously being extremely misunderstood. Luckily for you, you soon will understand these things!
1) The first of these groups consists of all people and creatures that have succumbed to the Scourge of the Beast, the mysterious plague devastating the town of Yharnam. To clarify what this means, Special Attack is defined as the poison on your weapon and the "extent that ATK is raised vs kin and severely transformed beasts." This means that even though the angry Yharnam villagers are all technically infected, only the "severely transformed" count as Beasts.
Physically, beasts are generally characterized by long fur or feathers, drastically increased size, and long claws. Basically if it's an animal or human that's been clearly deformed in some way physically it's a Beast for sure. In combat, Beasts are typically fast and are the most likely to sprint or jump at your character upon gaining vision (although enemies of all three types can do this). Beasts are generally also "dumb" compared to other enemies and usually have straightforward attack patterns. There are a few unique cases though.
The majority of the early bosses also share these traits and in many cases we're explicitly told they are Beasts. Jumpy Bridge Buddy, Hungry Dude, Dr. Pearl, and even our beloved Father Guacamole are classified as Beasts for obvious reasons.
2) As you move farther into the game you'll begin to encounter enemies that are less and less grounded in reality. Many people in this sub have aptly described The Kin of the Cosmos as creatures of true Lovecraftian horror, and this is a good way of thinking about them, at least as far as appearances go. The Kin are definitely identifiable because of this.
After killing Rob the Vacuum Liar you'll receive an item that was previously possessed by the "inhuman kin of the cosmos, brethren of the Great Ones." These three things clearly separate the Kin from other enemies. If they're human, they look like something that could exist on earth, or they don't resemble the Great Ones in any way, they're almost certainly not Kin. The vast majority of enemies in most of the final areas, including multiple optional areas, are Kin. The bosses within these areas are also prone to the same weaknesses.
3) And then there's pretty much just whatever's leftover. If it doesn't look even remotely werewolf or like a grotesque alien monster, it's probably just a boring old Neutral enemy. Not too much to say here. Sometimes it's really obvious they don't fit nicely into either category, sometimes it's hard to tell, and sometimes it just seems ridiculous for them to be neither. Multiple areas in the game, both required areas and optional areas, contain a majority of Neutrals, so that can make it easier to tell in some cases.
--Basics Everyone Should Know--
1) Beasts take increased damage from fire and are also scared of fire in many cases. When hunting in an area filled with Beasts, don't be afraid to use items, even if they seem rare. You'll encounter less and less Beasts as the game progresses, so these items are most valuable right around when you get them. You're more than welcome to struggle through some of the early bosses holding onto these items, but late game they're really just not as useful. Also be on the lookout for campfires. Enemies will take damage if they walk over fire, especially Beasts.
2) On the flip side, Kin take increased damage from bolt. Although there are items that can help with this, it's advisable to make a point of getting a bolt weapon or seeking out a blood gem that gives a weapon bolt damage. Some of the late game bosses can be breezed through with the proper weapon.
3) Neutral enemies do not have universal weaknesses or resistances. Not too much to say here. Some are weak to fire, some are strong to fire, etc. We're still trying to figure out exactly what each Neutral is strong and weak to. Work in progress!
4) Blood gems that give increased damage against Beasts or Kin are fantastic in general and should almost always be taken when you know you're up against one or the other, especially in the case of boss fight. I'll get into the math in future posts, but on physical damage weapons it's virtually always stronger to take the increased damage against a type instead of additional physical damage. The Beast/Kin damage gems are typically NOT better than gems that give increased fire/bolt damage if you have decent arcane. Beast/Kin damage gems are also often more valuable on lower durability weapons since the bonus damage does not decrease as much as weapons are used.
5) Arcane is absolutely incredible against Beast/Kin once you have weapons to take advantage of it. Although Strength and Skill definitely are more useful early on for damage, if you ignore Arcane entirely some fights will be rough. A lot of people dismiss Arcane builds as cheesy, and yeah, building Arcane right from the get go is silly, but it's extremely potent later into the game.
Note: Does the Saw Cleaver really do 20% bonus damage to Beasts? The answer is pretty interesting!