They can be found via the layer menu and the new adjustment command in Photoshop and there a number of them such as photo filter and LUT and hue and saturation, gradient map etc.
The key panel 'layers'- you can see everything and re-order them, disable them, delete them, duplicate them and much more to create a near infinite array of color manipulation
They modify any entries below. If there are images above them in the panel then they are not affected by that setting. Anything below is modified in some way. The reds, greens and blues (or cyans, magentas etc) can be changed in countless ways such as turning an image black and white or applying a change to the hue or levels of an image. The key thing about them is that they are non destructive, nothing has been modified below until you flatten the design. If you wish to apply all the effect as a destructive effect then just use the same effects in the image menu and if you change your mind to the result you can always use the history panel or the undo command.
As many as you wish, you can add multiple entries and you can re-position them everywhere and anywhere in the panel
it doesn't holding pixel information / images etc and as such it will not let you do certain things as there are no pixels to manipulate. It is an overlay that modifies the color of the underlying entries. You can re-size them as well and much more though this can only be done if you add the effect in a selection or add a design to the mask of the effect
You don't have to add them to the entire thing, you can use the selection tools and select part of the image and then go to the layer menu and new effect (such as black and white or gradient map or curves etc) and you will see the change only applied to part of the image selected by the selection.
At any point you can double click the entry in the panel and display the properties panel for this effect (such as curves and LUT and levels and black and white etc) and then tweak the settings for the red and green etc channels or change the input and output settings etc and as they are non destructive effects, you can always go back to the entry and double click at any time and change the settings.
You can also apply a feathering to the current mask for the effects using the properties panel and masks tab. You will then get a blurry edge to the effect (levels, hue and saturation, photo filter, LUT, gradient map etc)
It has blending modes as well as opacity etc, all the standard blending modes are there to be used and change the result such as difference, lighten, darken etc. You can set the blending by going to the top of the panel (with the entry selected) and changing the value to difference, darken etc.
You can apply transformations to them so instead of having a rectangular or circular, you can have a warped or scaled or sheared one. Which also makes it easier to add multiple effects (multiple LUTs as well as multiple black and white and curves etc) and re-position them with different warps or sizes etc - great for creating all kinds of interesting lighting effects added to faces etc
If you want the image to be the inversion of the current image, say you have a 'bright' effect over an image but you would prefer that part to be darker and the rest of the image to be brighter, go to the panel and select the thumbnail (second box on the entry) and go to the image menu and use adjustments and invert command
A mask is super great for stopping or showing the color effect. It is the second box on the entry, it is created when you add the entry. You can add brush strokes to it and gradients etc, in fact, anything is you wish and you will allow more or less of the color effect to change the image. If you go to the channels panel then you will see the mask channel and it can be manipulate there also
You can modify the image in countless ways by using LUTs, go to the lookup menu command and set a 3D LUT, abstract etc such as selecting blue tone, crisp / warm etc and you can radically change the image.
You don't have to apply the effect to all the entries below, you can clip them to the current selected entry by going to the bottom of the panel and click the first button in the properties panel. You can tweak all the other settings such as setting the red and blue etc but clicking the clip means only that entry changes and all the others are left untouched. If you want to reverse that, go to the properties panel and click the button again