Where can I find a hardware manual (or documentation) for Arduino board?
The Arduino Uno is little more than an ATmega328P microcontroller. In
most cases you will be interested by the documentation of the
microcontroller itself. Anyway, here are both:


The Arduino Uno product page contains the most basic specs, as well as links to


the board schematic
the pin mapping, but see below for a nicer alternative

the ATmega328P product page links to


the ATmega328P datasheet, a 650 page authoritative document
the AVR instruction set manual, for those who want to play with assembly
many application notes discussing specific uses of some peripherals



Alberto (aka PighiXXX) has published nicer versions of the Arduino Uno
pinout.