It is simply a file with parts in it.
To take an example, I followed the link given in a mod thread and ended up in GitHub.
There are 2 files and 1 folder. For this custom parts pack, we just need to download the .pack file, put it into the folder indicated in the tutorial you read.
It's important to read the readme or other instructions included with a part pack to see if there are any special steps, like installing something else such as an electricity mod.