What's the difference between "to and fro" and "back and forth"?

I'll give you an example from "The Mask of The Red Death" by Edgar Allan Poe:

"Its pendulum swung to and fro with a dull, heavy, monotonous clang;"

How does that differ from:

"Its pendulum swung back and forth with a dull, heavy, monotonous clang;" ?

How frequently "to and fro" is used these days?

It doesn’t really mean anything different. The archaic fro occurs only in that phrase. It used to be by far the most common way to say that, but no longer is: see this Google N-gram:

Nonetheless, to and fro still occurs fairly frequently, so is by no means uncommon.

In the example you cite, to and fro is essentially identical in meaning to back and forth.

However, to and fro can also carry the less specific sense of ‘moving about in different directions,’ similar to here and there, as in

The lambs frolicked to and fro on the grassy hill.

To and fro is alive and well in present-day English usage, but it sounds somewhat antiquated and may seem unduly whimsical if used in a serious context (few today would use it to describe the motions of a torture implement). There is also a noun form of the phrase, usually hyphenated (to-and-fro), which means debate or vacillation:

There's been an awful lot of to-and-fro about the appointment of the club's new treasurer.