Although many non-Japanese think sushi is the national food of Japan, I would rather say tempura is the Japanese national food.
Tempura was inspired and adapted from European fritter in the 16-17th century. In Tokyo(it was called Edo at that period), tempura was a street food, sold at food stalls. This is also similar to sushi, which was also a street food during Edo era.
I assume that tempura was sold outside to prevent from fire. Edo was frequently damaged by large fires, and tempura is one of the main causes of outbreaks of fires still now in Japan. It is told that 3 main causes of fire in Japan are, tempura(due to using a lot of oil and heat them up to high temperature), cigarettes and kerosine stoves.
Tempura can be vegetables; major ones are onions, green peppers, sweet potatoes, pumpkins/squashes, lotus roots, egg plants, and carrots. Mushrooms and sansai; mountain vegetables are used to with various seafood. Seafood can be fish(white meat), shrimps, prawns, cuttlefish, squid, crabs, scallops and conger eels. Hardly ever see meat(cattle) tempura and sometimes chicken tempura.
Yasai tempura means vegetable tempura, Ebiten means prawn tempura. Ten is used for abbreviation of tempura. Tempura teishoku, is a menu including mix tempura, rice, miso soup and several small dishes. Tendon is tempura on rice with soy sauce base sweet sauce, tempura soba or tempura udon is soba noodle or udon noodle with ebiten(prawn). Tenzaru is tempura with cold noodles. If you order tenzaru, you will get two similar sauces. One is for tempura and the other one for the noodles. Tempura sauce can be lukewarm but noodle sauce should be chilled. Then, usually tempura sauce is much light color than the noodle sauce(which means noodle sauce contains more soy sauce=saltier). There are many people who just pour soy sauce on the tempura, rather than dipping into the tempura sauce.