All information here is related to 2012 version of Google Nexus 7 manufactured by ASUS. I don't take any responsibility for bricked or destroyed hardware. Also, some actions here may void your warranty. You have been warned!

99% of information here can be found on intranets. These are, pretty much, personal notes. Respect to all those people for all the work!

Speaking of which, here are some links:

• unlocking boot loader by grey_olli
• replacing recovery image by grey_olli
• rooting nexus 7 summary by dmmarck with links to guide and video. Beware, though, as it has obfuscated links for non-registered users(which I think, sucks)

Detailed information about hardware can be found at wiki. I see no reason to duplicate this information.

• CPU - Nvidia Tegra 3 T30L ARM Cortex-A9, quad-core, 1.2GHz
• GPU - Nvidia ULP GeForce, twelve-core, 416MHz

<note important>Unlocking boot loader will wipe all your data. Therefore, you're advised to backup your data before proceeding further.</note>

• you need Android SDK to get adb and fastboot. Look for adt-bundle-linux-x86_64. Note, however, despite it's 2013, “compat” libs are still required by 64bit version(what a shame!). This is covered by installing ia32-libs on Ubuntu. I don't know about other distros.
• enable USB debug mode by:
  • go into Settings → About tablet and click on Build number seven times
  • go into Settings → Developer options and enable USB debugging
• # ./adb devices

List of devices attached 
015d46d948101e11        device

• # ./adb reboot-bootloader or, I guess, this would do too: power off Nexus 7, hold simultaneously Volume down and Power buttons
• # ./fastboot oem unlock
• press Power button to unlock boot loader