I know about adjustment layers and filters are and the differences between them. But now I discovered this new concept called "layer styles" like layer shadows and stuff.
My question is: What exactly are layer styles and how do they differ from layer filters and adjustment layers?
I mean, we can drop a shadow using filters too right? Then why make an entirely new concept called layer styles?
A layer style contains multiple effects that can be saved as a preset and re-applied for multiple layers (or groups or layers) at once. For instance you can have a drop shadow, a bevel, a stroke, a gradient etc, all included in a layer style. There are alternatives of course, but using a layer style makes it quick to do repeated processing on multiple images.
Think about an online shop which has 1,000 pictures of watches to process in exactly the same way (eg. the same shadow and the same glow on the product). One option is to use a layer style for all these.
Adjustment layers address totally different image "effects". The subject is so vast that explaining all the differences could fill up a book. Please read the official Adobe docs to understand this fully.
Layer style = a processing that is applied to a layer before it is shown. For example the drop shadow. NOTE: the original layer content stays intact, but hidden. Only the affected content is shown. This makes possible to remove the effect or to adjust its parameters
Adjustment layer = a processing receipe that is applied to the underlying single or multilayer image before the image is shown. For example you can add a brightness and contrast adjustment layer. NOTE: the adjustment layer do not affect permanently until merged with underlying layers. This makes possible to adjust the effect parameters or to remove the effect. Adjustment layer can have a layer mask.
The difference: Layer style must be assigned to a layer. The style is not itself an item in the layer stack. Adjustment layer appears as an item in the layer stack and affects the image that lies under the adjustment layer. The set of possible adjustment layers is different than the possible layer styles.
Same: Non-destructive
ADDENDUM due the comment: You can't create the same effect as a layer style and as an adjustment layer. See the lists, what is selectable. For example there exists no layer style "Brightness and Contrast" and there's no adjustment layer that creates drop shadows. Why? The reason is unknown. I'm ready to rebuy my copy of Photoshop, if Filters and layer styles were available as adjustment layers with layer masks.
Filters are destructive processes that affect only to the current layer which you have selected to be under your work just now. That layer must contain some bitmap image, not paths or texts or adjustments or smart objects. You can't define a mask to reduce the filtered area or the depth of the filtering. But you can make a marquee selection that defines the affected area. (layer style applies to the whole layer)
I recommend to make a spare duplicate of the layer before applying a filter. A save is also a good idea, because some filters are so heavy loads that the computer can get stucked.
There is no need to organize things the way they are organized in Photoshop. Many of these have no particular contemporary reason for being done the way they are.
For example there is nothing a adjustment layer can do that a filter can not. There is really no reason why a filter layer couldn't make a drop shadow or a emboss etc.* Mainly the difference is that they represent a different step in the evolution of the Photoshop GUI.
So once upon of time there were no nondestructive edits. There was a filter infrastructure which were destructive. Then they decided to add effects for Layers. Then they decided that you might want to adjust multiple things non destructively and then they decided that yes you could let any filter apply filters on layers non destructively (you can apply filter to smart objects that are nondestructive).
As a result they have different GUI for things that could be filed under one GUI item. These have some differing capabilities too because they were separately designed. But this is just how reality works things evolve. They could have made a similar design choice as After Effects or say nuke/fusion and then they wouldn't have the need to differentiate between the 3 things. But it works, kind of, no need to fix what is not broken.
* In fact back in the day you used to be third party filters that did these things.
