As the Torah recounts, God commanded the Jews to create this special unleavened bread. During Passover it is eaten as a flat, cracker-like bread or used in dishes as breadcrumbs and in the traditional matzo ball soup.
Matzo that is kosher for Passover is limited in Ashkenazi tradition to plain matzo made from flour and water. The flour may be whole grain or refined grain, but must be either wheat, spelt, barley, rye, or oat. Some Sephardic communities allow matzos containing eggs and fruit juice to be used throughout the holiday.
Passover and non-Passover matzo may be soft or crisp, but only the crisp "cracker" type is available commercially in most locations. Soft matzo, if it were commercially available, would essentially be a kosher flour tortilla.
Non-Passover matzo may be made with onion, garlic, poppy seed, etc. It can even be made from rice, maize, buckwheat and other non-traditional flours that can never be used for Passover matzo. Gluten-free matzo-lookalike made from potato starch, tapioca, and other non-traditional flour is available and may be eaten on Passover, but does not fulfill the commandment of eating matzo, even for people with celiac disease who cannot eat Passover matzo, because matzo must be made from one of the five grains (wheat, barley, oat, spelt, and rye), all of which contain gluten, except for most (but not all) types of oat matzo. Oat matzo may only be used by those who cannot have any other kind because it's not certain that oat is actually one of the five grains (it may be a mistranslation), so those who can have wheat matzo should do so.