They're not interchangeable — you need to purchase a different Pencil depending on what version of the iPad you have. Both Pencils are current models, since there are a number of iPads currently in use which make use of both accessories.
The original Apple Pencil is gloss white, completely round, and features a silver ring near the rear. Behind the ring is a removable cap that gives you access to the Lighting port to charge the pencil. Again, it costs $99 from Apple.
The Apple Pencil (2nd Generation) has a less shiny matte white finish, and is flat on one side so it doesn't roll as easily. It has no silver band in the back, and instead of a Lightning port, it charges wirelessly when it's attached to the iPad. This Pencil is magnetic, so it sticks to the iPad for storage and charging. It costs $129 from Apple.
Apple offers the option of engraving a short text message (such as your name, initials, or phone number) into an Apple Pencil (2nd Generation) for free when you order it online. You can't engrave the original Apple Pencil.