It's not something that can be answered in a sentence or two, I would suggest downloading a special site or buy a book. I've grown them for years and still don't fully understand. Fortunately there are some on here who know a lot so I'll read their replies too.

I have some clematis some you cut back spring some you don't have to what is the name

It depends which clematis you have as to when and how you prune.

Would help no end if you knew the name of your Clematis, or failing that when it flowers. General rule is If it flowers before June don't prune!

I wrote a blog in June on this very subject, Di. If you click on my photo, it will take you to my homepage. You'll find the blog in the June archive. I hope it helps. :-)