Senbei is a cracker made from grains such as rice, and wheat. Most ones are called beika-senbei (米菓煎餅) which use rice grains squished and flatten, and the rice dough will be either baked or fried. The baked one is called yaki-senbei (焼き煎餅), and the fried one is called age-senbei (揚げ煎餅). The common seasoning used are soy sauce and salt, and there are shops that bakes and sell fresh senbeis.
It is believed that the oldest senbei is soka-senbei from Soka city, Saitama prefecture. The first senbei was made by an old lady named Osen. She was running a teahouse which served dumplings. The dumplings were popular, and a samurai who visited the teahouse suggested to make the dumplings flat and bake them.
The farmers who lived in Soka city also made squished, and dried rice snack which are seasoned with salt. As the city evolved as a post town, the rice snack started to be sold as a product, and became popular in many different places. After then, soy sauce, which was produced near Tonegawa river, became a standard seasoning for soka-senbei.
There are also other famous one coming from different regions. For example, there is nure-senbei which means wet one from Choshi city, Chiba prefecture. This kind of senbei is made by soaking the senbei in say sauce right after it is baked.
There is kawara-senbei which is sweet and is hard. It uses wheat flour, egg, and sugar, and the shape imitates roof tile. It is considered more as a Westernized fashionable sweets. Katayaki is the hardest one, which is also a hard type of senbei from Iga region of Mie prefecture. It was a portable food carried by ninja.