p st.inspect is short for print st.inspect, which sends the string to STDOUT.
Not quite.
print "blah" prints blah to stdout
print "blah","bloo" prints blahbloo to stdout
puts "blah" is short for print "blah", "\n"
p st is short for puts st.inspect
These are all documented in the Kernel module:
http://www.ruby-doc.org/core/classes/Kernel.html
Well, except for the #inspect method, which is documented in the Object class, as well as, probably, other places that override its behavior.
http://www.ruby-doc.org/core/classes/Object.html
Granted, that's not the best way to learn the language. 'Programming Ruby' is a much better way, or why's (Poignant) Guide to Ruby.
http://phrogz.net/ProgrammingRuby/
http://poignantguide.net/ruby/
HTH,
Devin