Anko (あんこ) is the catch-all term for various products based on azuki beans
boiled with sugar. Also simply called an (あん) and often used as
a prefix, e.g. anpan is bread with anko, anmitsu
is mitsumame topped with anko, etc. Anko has distinctive
taste, which some (including yours truly) find addictive but some
loathe with a vengeance.
Anko is made by boiling off the extra water from
yude azuki (azuki beans boiled in sugar) and mashing what remains.
The unfiltered mash is known
(lit. "lumpy an"). If put through a fine strainer, discarding
about 1/3, it becomes koshian (filtered an), also known as
nerian (kneaded an). The resulting paste can be dried, in which
case it becomes
sarashian (dried an). Sarashian is useless in itself, but it
keeps almost forever and can be easily reconstituted into koshian
simply by adding water.
Whereas anko is by default made from red azuki beans,
it is also possible to use white beans (esp. hakkatô,
white flower beans). The resulting paste is
known as shirokoshian (white filtered an). Shirokoshian
is rather tasteless in itself, but due to its lack of flavor and
color it can be mixed with a wide variety of other foods, such as green
matcha powder, yellow kabocha pumpkin, satsumaimo yams,
even miso paste.
Note that ankô (鮟鱇), with a long O, is Japanese for
angler fish, a popular stew ingredient in winter.