I am going to make a compiler for C (C99; I own the standards PDF), written in C (go figure) and looking up on how compilers work on Wikipedia has told me a lot. However, after reading up on lexers has confused me. The Wikipedia page states that:

the GNU Compiler Collection (gcc) uses hand-written lexers

I have tried googling what a hand written lexer and have come up with nothing except for "making a flowchart that describes how it should function", however, isn't that how all software development should be done?

So my question is: "What is a hand written lexer?"