Difference between spider vs. crawler?

Hi,
I am a Java programmer on the Front-end, and now I received a task that
implies knowledge about search engines. I have neither idea how they
work, nor what are the principles and ideas behind this concept
For example: what is (in plain English) the difference between a
"spider" and a "crawler"?

Do you also know some URLs as good tutorial in understanding the basics
of search engines and how do they work?

Thank you very much for you answers and have a nice week-end  :-)
John