I have an image texture with a couple of parts that I'd like them to be semi-transparent. How can I achieve this? I know how to to make the whole texture semi-transparent and control it's transparency, but how can this be done for only a few parts of the texture? In photoshop these parts aren't semi-transparent but I could make them, if it's necessary.
The general way you want to do this is by using the mix shader node to mix between your "solid" shader output (diffuse, glossy, principled, any shader you want) and a transparent chader node. By plugging the alpha channel of your image into the "Fac" setting of the mix shader node, the alpha channel of the image lets you control how transparent the result is. The alpha channel is what changes when you make parts of your image transparent in Photoshop. You can also use nodes like the noise or Musgrave texture nodes to procedurally generate transparent and opaque portions of the object. Just plug the factor output to the factor input on the mix shader node.
See this post for an example.
You can use a MIX shader to mix a transparent, glass or translucent shader with some other opaque shader. Then you can use a texture as the factor input to the mix shader.
This is an example, here I have a glass material that mixes a gloss shader for reflection, the output of the mix is then passed to another mix shader, which mixes the reflective glass material with an opaque black glossy shader, then I use a checker texture as the factor input to that mixer. Now I get checkered black and blue glass.