You can find the Photoshop displace filter in the distort category in the filters menu.
It moves pixels depending on the underlying source map, and you can choose a PSD files to create a strong or subtle shift of those pixels in any image.
You can use PSD files as displacement maps. Use them as normal files but you can also use them with many other tools in the application. You can set the value to be positive or negative in the scale to change the direction of the effect. If you want to use them in other applications then you will have to convert them to PNG etc
To use the maps, go to the effect panel and browse for the file and then enter the horizontal scale, 10% or 20% etc or so for an extreme setting of 100% etc to really blast the image either up or down or diagonally. You can also set the 'Stretch to fit' (matching the source map to the image) or use tile where the map is tiled, though the result may not be what you want depending on how seamless the design is. You can also wrap around or use the repeat, personally, I always use the wrap around where the pixels from are moved from side to side. It is slightly odd in that there is another panel after the first requesting a PSD file and you have to select one or cancel.
The effect is one of those tools that probably works best when applied multiple times. One application can look a little limited, apply it again and the magic really happens and even better, apply it combined with others such as noise or oil paint and you will see the tool in many new ways. You can also combine them with other resources as well (they are not cast in stone) such as adding addition artworks to the design (perhaps intermixed with effects) such as brush strokes
As with many effects, I find that many work best with a layer so before you use one, simply duplicate the current layer via the layer menu. Apply the effect to that layer and then use the layer menu and the blending modes to blend with the original image such as select blending mode of darken or multiply etc. Open an image and go to the layer menu and duplicate layer (perhaps more than once if you want multiple layers to blend) and then go to the panel and set the scale, repeat etc as well as the PSD file to use to modify the pixels. You can then use the blending modes in the layers panel to mix the designs
Another option would be to use the same design but with the top layer reduced in size and the effect applied to that and you will then get an interesting transparency effect and you can then add layer effects / styles such as bevels and shadows
A key thing for me is that it can be used smart objects and that means instead of just applying the effect and that is that, you can use it but at the same time you can decide that you don't want it or try different settings and so on. It is a pity that you can't select the effect entry and duplicate and duplicate (so adding 10 or so entries), you have to manually create them. Select a layer and layer menu and smart objects and convert (to smart object) and then apply the effect and then apply another and so on. Add adjustments to smart objects to re-color the design as well as use additional smart effects. You can also combine multiple smart effects with the smart object and also combine those effects with adjustments and layer effects to create a truly awesome effect. You can also bundle the whole thing up in a new smart object and save the result to the CC libraries.
Makes it a lot easier to create a PSD map file, apply shapes and brush strokes and effects and you can then see the design tiled. You can then save it as a PSD via the file and save command.
It can be used with color channels, separate color channels. Go to the Channels panel which is found via the window menu and select red channel and then apply the effect as normal and use a number of settings say 10% for the scale. Go to the green channel and then use perhaps 20% and then to the blue channel and then use 30%. The end result will be an unusual jarring colorful image
You can also open the PSD files and apply other effects to them such as blurs etc. If you want to avoid the seams then use masks / selections etc to control the effect on the edge. Or perhaps use brushes on the map and only modify the center of the work. Another option is to use the edit menu and define pattern. You can then re-size the document, perhaps make it 10 times bigger and then fill that new document with patterns and they will still be seamless. You can then save that new file as a PSD file and use that with the effect. Open PSD file and edit menu and define pattern and layer menu and then new fill layer and pattern and select a pattern from your pattern presets and then set the scale to 25% and flatten and use twirl or another and then save as PSD format. It should be noted that some effects will damage the edge of an image and the resulting displace file will not be seamless
You can apply the effect and distort the image but you can always fade the result. Go to the edit menu and fade command and then use a lower opacity as well as a different blending mode and click OK