Consider the string 011 over the binary alphabet. All the prefixes, suffixes, and substrings of this string are listed below.
Prefixes: $\varepsilon$, 0, 01, 011.
Suffixes: $\varepsilon$, 1, 11, 011.
Substrings: $\varepsilon$, 0, 1, 01, 11, 011.
Note that x is a prefix (suffix or substring) to x, for any string x and $\varepsilon$ is a prefix (suffix or substring) to any string.
A string x is a proper prefix (suffix) of string y if x is a prefix (suffix) of y and $x\neq y$
In the above example, all prefixes except 011 are proper prefixes.