The general consensus seems to be that iodized table salt (such as Morton's in the US, usually comes in a cylinder with a spout on top) is questionable. Some people say it's okay to use, and the fish can benefit from the iodine, others say that you're just adding more chemicals to your tank.
Kosher salt, however, is widely accepted, very inexpensive, and easy to find (in most parts of the US, at least). Diamond Crystal seems to be the most common brand around here. It's pretty cheap and has the benefit of being useful in the kitchen as well -- not something I'd do with aquarium salt!
Kosher salt (in my experience) tends to come in a rectangular box (like a cereal box) with a spout on the side. It should be marked with something along the lines of "suitable for koshering", or the ever-subtle "Kosher Salt", and it may be found in the kosher food section of your grocery store, as opposed to the cooking area.
Keep in mind that kosher salt doesn't have the anti-caking agents that table salt does. In other words, it might clump up, particularly if you keep it in a moist area, like the kitchen. The clumping doesn't hurt anything; you can just squeeze the box a bit to break it up before pouring.
And remember -- never pour salt directly over whatever you're adding it to, whether it's your aquarium or your batch of cookie dough. It's much safer to measure in a "neutral" area, so that you don't inadvertently overdose!
Good Luck,
Pamela