Most deep fried Japanese food has fairly recent origins in that it first appeared in Japan after the country opened up to the West in the late 1800s. Tempura on the other hand, whilst inarguably of Western origins, has a surprisingly long history dating back to the 16th century when it was first introduced by Portuguese traders. It was a food mostly restricted to the wealthy at first due to the expense of oil but by the Edo era, it had spread to the common people as a street food before developing gradually into what we have today.
Tempura consists of a variety of seafood and vegetables deep fried in a simple batter made of flour, eggs and cold water. The batter gives the food a light, crispy texture that complements the texture and flavour of the food inside.
Tempura is usually seasoned with salt or served with a dipping sauce made of dashi, mirin and soy sauce (tentsuyu). Some places might also serve it with a small dish of matcha (powdered green tea) and salt for dipping.
Tempura can be served as either a side dish or a main dish and there are many restaurants in Japan that specialise in it, ranging from the moderately priced to high end.