Sometimes they're interchangeable, sometimes they're not.
At the other end of the spectrum, we have dashes. If you want to hang a spotlight on your words, decorate them with dashes. You can use dashes the same way we just talked about using parentheses, to enclose fragments or whole sentences, but you'd better be sure your words are worthy of dashes. Dashes interrupt your sentence in a way that parentheses or commas don't. Here's an example:
They fled through the woods, and then George—dear, sweet George the accountant—jumped out from behind a tree and stabbed them.
It's appropriate to interrupt that sentence with dashes to remind the reader that the attacker has unexpected qualities—that he's dear, sweet George the accountant.
But this is English, so there's an exception to the dashes-are-dramatic rule. You can use dashes in a mundane sentence when the part you need to set off already has commas, like the date we enclosed in parentheses earlier. You could write
The 30th anniversary of the eruption of Mount St. Helens—May 18, 1980—brought back vivid memories of ash and darkness.
Just remember that when you use dashes instead of parentheses, you're highlighting the date instead of simply noting it or providing it as background information.
Dashes are the yowl of a pirate dashing into a fray.
Another difference between parentheses and dashes is that you always have to use two parentheses, they always enclose something, but it's fine to use one dash alone to introduce an important or exciting statement, or a statement that already has commas in it. You could write
There was only one thing missing from the pirate ship—pirates.
That dash is appropriate because the announcement that the pirates are missing is probably important or dramatic. In a sentence like that, where something is defined or expanded, you're choosing between the dash and a colon. You could just as properly write
There was only one thing missing from the pirate ship: pirates.
That sentence just doesn't have the same wild feeling as the sentence with the dash. A colon is a more stoic, buttoned up punctuation mark than a dash.
And as before, you can also use one dash to introduce a longer, pedestrian statement if the statement already contains commas.
Next: How to Use Commas