Radical are used in Chinese dictionaries and Kanji-Japanese dictionaries to order characters in sets by the number of strokes they contain.
Full characters are ordered according to their initial radicals, which fall into roughly 200 types. Then these are subcategorised by their total number of strokes.
The commonly used system, known as the "radical-and-stroke-count" method, makes use of the fact that Chinese characters (total number very approximately 5,000-50,000) are assembled from some 1,000-2,000 subcomponents. Of those subcomponents a few hundred are designated as "radicals" for indexing purposes. The most commonly encountered radicals are very basic ideograms, representing simple concepts like "man (人)," "sun (日)," or "tree (木)". The steps involved in looking up a character are (a) identify the radical under which the character is most likely to have been indexed; (b) find the section of the dictionary associated with that radical; (c) count the number of brush or pen strokes in the non-radical portion of the character; (d) find the pages listing characters under that radical that have that number of additional strokes; (e) find the appropriate entry or experiment with different choices for steps a and c.
For example, (信) (man standing next to his words(mouth with sound) meaning truth, faith, fidelity, sincerity, trust, confidence, reliance, or devotion. This particular character is composed of the radical for "man," and 7 additional strokes. To look this up one by the multi-radical method,  (http://www.csse.monash.edu.au/cgi-bin/cgiwrap/jwb/wwwjdic1R) one finds the radical for "man" in the dictionary (most dictionaries will have an index page to help with this step) and then passes through 1 additional stroke, 2 additional strokes, etc. until one finds entries for 7 additional strokes. If the radical chosen by the user matches the radical used by the dictionary compiler (not a problem in this simple example, but it can be tricky), and if both user and dictionary compiler count strokes the same way (occasionally a problem), the entry will be found.
This principle of categorisation is exploited by everybody who must learn to write Han characters: The vast number of Chinese characters can be much more easily memorized if they are mentally decomposed into their constituent radicals. The question of how many characters there are is a subject of debate.
In the 18th century, European scholars claimed the total tally to be about 80,000. This number, however, is exaggerated, as the most comprehensive dictionary (the Kangxi Dictionary 康熙字典) lists about 40,000 characters. One reason for large number of characters is that they include all of the different characters in the different variations of Chinese. Popular estimates say that about 3,000 characters are needed to read a Chinese newspaper, and 4,000 to 5,000 constitute a decent education.
Often a character which is not commonly used will appear in a personal or place name in Japanese and Chinese names (see Japanese name and Chinese name). This has caused problems with some computer encoding systems which include only the 5000 or so most common characters and exclude the less often used characters. For example, the Taiwanese politician Wang Jian-hsüan has a name that is difficult to encode in some computer systems because the last character in the name is a uncommon character.
strokes. The list is from Radical-Stroke Index of Unihan Database  (http://www.unicode.org/charts/unihanrsindex.html)
one, line, dot, slash, second and hook.
two, lid, man, legs, enter eight, down box, cover, ice, table, open box, , knife, power, wrap, spoon, right open box, hiding enclosure, ten, divination, seal, cliff, private and again
mouth, enclosure, earth, scholar, go, go slowly, evening, big, woman, child, roof, inch, small, lame, corpse, sprout, mountain, river, work, oneself, turban, dry, short thread, dotted cliff, long stride, two hands, shoot, bow, snout, bristle, step
heart, halberd, door, hand, branch, rap, script, dipper, axe, square, not, sun, say, moon, tree, lack, stop, death, weapon, do not, compare, fur, clan, steam, water, fire, claw, father, double x, half tree trunk, slice, fang, cow, and dog
profound, jade, melon, tile, sweet, life, use, field, bolt of cloth, sickness, dotted tent, white, skin, dish, eye, spear, arrow, stone, spirit, track, grain, cave, and stand.
bamboo, rice, silk, jar, net, sheep, feather, old, and, plow, ear, brush, meat, minister, self, arrive, mortar, tongue, oppose, boat, stopping, color, grass, tiger, insect, blood, walk enclosure, clothes, and west.
see, horn, speech, valley, bean, pig, badger, shell, red, run, foot, body, cart, bitter, morning, walk, city, wine, distinguish, and, village.
gold, long, gate, mound, slave, short tailed bird, rain, blue, and, wrong.
face, leather, tanned leather, leek, sound, leaf, wind, fly, eat, head, and, fragrant.
horse, bone, tall, 髟 hair, 鬥 fight, 鬯 sacrificial wine, 鬲 cauldron, and, 鬼 ghost.