What is the difference between early career research positions
Researcher may be a permanent position (or not). Post docs are usually term limited and possibly renewable, but not likely indefinitely. 

But I imagine that different institutions may use them interchangeably for things that are pretty similar. 

Some postdocs actually do a bit of teaching and advising, and in that case, might not be best described as "researchers", but I don't think there is any universally valid terminology for these things. 

But each offering announcement should make clear what the expected duties are and, how free you will be, or not be, to carry out your own research.