Prefixes and suffixes are sets of letters that are added to the beginning or end of another word. They are not words in their own right and cannot stand on their own in a sentence: if they are printed on their own they have a hyphen before or after them.
Prefixes are added to the beginning of an existing word in order to create a new word with a different meaning. Suffixes are added to the end of an existing word.
In this marvellous Oxford blog you can analyse the different aspects of these set of words and get rid of the common problems you come up to when we deal with them.
To go to the main page of prefixes and suffixes in the blog: Prefixes and suffixes
For more particular aspects of prefixes and suffixes:
Do you know your -ibles from your -ables?
If you want to practice with some exercises online: Prefixes and suffixes exercises