I am trying to understand various aspects of spot colours. If I am working in photoshop and I use say a pantone colour for a part of my artwork (in my CYMK file) ... lets suppose I've used it as the colour for a logo which is overlaid on top of a full colour image ... and then export this as a PDF and give to my printer ( who is using an offset printer) ... what will happen? I cant imagine that the pantone will print as intended - how would the printer know that I had used a spot colour? Would the pantone be approximated as CYMK? How would I do something like this properly?
You typically wouldn't apply spot colors within the PSD file, though you can. There are various ways to do this.
One would be as part of a duotone image. In which case you'd set up your image to print as a duotone and use the spot color as one of the colors.
Another option is to create a new channel and apply the spot color to that channel. And communicate to your printer that you are going to use a spot color.
If you simply choose a spot color but print as CMYK, the pantone color chosen will be converted to CMYK--likely to varying degrees of success depending on the particular color you wanted to use.