Henna is known as a natural hair dye with conditioning properties. It comes from a plant and real henna is only red-orange. Other natural hair dyes are marketed with or as henna if it is not red-orange it is not henna or has an additive natural or otherwise to change the color.
There are 2 ways to do henna(IMHO):
The hard way: Buy henna powder, mix with natural additives, let sit overnight, apply to hair
The easy way: Buy LUSH henna bar, grate, add hot water, apply to hair
Most of the things people add to henna are to help the color or prevent dryness. LUSH hennas called CaCas are henna powder mixed into shea and coco butters you can still add things to help the color if you want. Henna also relaxes your curl pattern slightly. I saw no difference in my curl pattern post-henna but individual results vary.
Here's me doing LUSH henna:
Here is IttyBittyButSexy doing traditional henna:
And IttyBittyButSexy using LUSH: