A suffix is a word part that can be attached to the end of a base/root word to form a new word.  It has its own definition but can not stand-alone.

Additionally, unlike a prefix, a suffix changes the part of speech and sometimes the meaning of the base/root word to which it is added.  Analyzing the base/root word and the suffix together create one new definition for a word. For example, the word reporter.

                                report + er

A report is a detailed document of a factual matter.  The suffix  '-er'  means 'one who, a person associated with'.  Thus, the word reporter means 'one who gives detailed information of a factual matter.'   

Note:
Sometimes a word will contain both a prefix and a suffix.  For example, counterclockwise.
  
                          counter + clock + wise
 
Counter- is a prefix meaning 'contrary/opposite.'  Clock is a device indicating time by means of hands moving on a dial.    -Wise is a suffix meaning 'showing how.'  Thus, counterclockwise means 'showing the opposite direction of the hands moving on a clock.'

There are two kinds of suffixes: inflectional and derivational.