The basic part of any word is the root; to it, you can add a prefix at the beginning and/or a suffix at the end to change the meaning. For example, in the word "unflattering," the root is simply "flatter," while the prefix "un-" makes the word negative, and the suffix "-ing" changes it from a verb into an adjective (specifically, a participle).
English itself does not use prefixes as heavily as it once did, but many English words come from Latin, which uses prefixes and suffixes (you can use the word affix to refer either to a prefix or a suffix) quite extensively. For example, the words "prefix," "suffix," and "affix" themselves are all formed from "fix" by the use of prefixes:
* "ad" (to) + "fix" (attached) = "affix"
* "pre" (before) + "fix" = "prefix"
* "sub" (under) + "fix" = "suffix"
Note that both the "-d" of "ad" and the "-b" of "sub" change the last letter.
Here are some of the most common Latin prefixes (for the meanings of the Latin roots, look up the words in a good dictionary):
(away) abrupt, absent, absolve
(to) adverb, advertisment, afflict
(not) incapable, indecisive, intolerable
(between, among) intercept, interdependent, interprovincial
(within) intramural, intrapersonal, intraprovincial
(before) prefabricate, preface prefer
(after) postpone, postscript, postwar
(under) submarine, subscription, suspect
(across) transfer, transit, translate