There is a new German Rails-book out there: rapidwebdevelopment.de. I haven't read the book, but they have free PDF "Ruby-Basics" in German available for download.

That PDF got the first explanation about symbols I understood:

"In Ruby you often use symbols instead of strings. [...] Symbols are atomic, so that there is only one instance for the same symbol. A string "ruby" can be instantiated a million times in a programm, a symbol :ruby only once: "ruby".objectid # 443352 "ruby".objectid # 443332 :ruby.objectid # 880910 :ruby.objectid # 880910

Symbols are used if you don't need instance, e.g. as Keys in an Hash or in the decleration of names:
hash1 = { :name => "Thomas", ... }
has_one :motor

The comparison of symbols is much faster than the comparison with strings. [...]