Source: http://www.linguistics.ucla.edu/people/schuh/lx001/Web_Quizzes/Quiz_03/03web.html
Morphemes - the building blocks of morphology
There are two types of word. The first type is called simple words. Simple word don’t have an internal structure and only consist of one piece, like work. There is no way we can divide work (wo-rk?) into smaller parts that carry meaning or function. Complex words however, do have an internal structure and consist of two or more pieces. Consider worker, where the ending –er is added to the root work to make it into a noun meaning someone who works. These pieces are called morphemes (the smallest meaning-bearing units of language).
A simple word only consisting of one single morpheme is therefore a free morpheme, that is, it is a word itself. Examples are house, work, high, us and to. Morphemes that must be attached to another morpheme to receive meaning are bound morphemes. If we break the word unkindness into its three morphemes un-, kind and -ness, we get two examples of bound morphemes: un- and -ness, as they require the root kind to make up a word.
These are also called affixes as they are attached to the stem. The affix un- that go to the front of a word is a prefix and -ness that goes to the end is a suffix.There are also infixes and circumfixes. However, these two types are not common in English language.
Pictures taken from:https://sites.google.com/a/sheffield.ac.uk/all-about-linguistics/branches/morphology/what-is-morphology
