Most often different names for different plants, though a few people occasionally use both names for one plant for some odd reason, apparently (see this page on thimbleberry that also suggests salmonberry as a common name). In 99.9% of the instances I've seen though, thimbleberry refers to Rubus parviflorus and salmonberry refers to Rubus spectabilis.
Thank you, I thought they were two different plants but spoke with some whom suggested that they were one and the same.
