So if I download and play a mod does that affect my current characters and if so is it permanent?
I’m a newbie to this forum and modding so sorry if this is a silly question…
No, it does not affect your existing characters. Unless you jump through some minor hoops, you will need to create a new char to play the mod and cannot play it with your existing chars.
When playing a mod with a char, whether there are permanent changes will depend on the mod and the char, so there is no definitive answer.
If e.g. you choose a mastery from the mod then yes, your char is dependent on that mod. If your char however has some items from that mod but uses the vanilla masteries, he could play another mod that also has vanilla masteries, but would then lose the mod-specific items when you switch to a different mod. So the former (mod specific mastery) is permanent while the latter (mod specific item) is not.